diff --git a/Cargo.toml b/Cargo.toml
index 6cd9239..7d55f07 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 name = "bibliographix"
 description = "A bibliography management crate."
-version = "0.4.0"
+version = "0.5.0"
 authors = ["trivernis <trivernis@protonmail.com>"]
 edition = "2018"
 license = "Apache-2.0"
diff --git a/src/bib_manager.rs b/src/bib_manager.rs
index 5cda345..3e0096a 100644
--- a/src/bib_manager.rs
+++ b/src/bib_manager.rs
@@ -99,7 +99,7 @@ impl BibManager {
 
                     entry_map.insert(K_KEY.to_string(), k.clone());
 
-                    Some(*BibliographyEntry::from_hash_map(&entry_map)?)
+                    Some(*BibliographyEntry::from_hash_map(&entry_map).ok()?)
                 })
                 .collect::<Vec<BibliographyEntry>>();
 
diff --git a/src/bibliography/bib_types/article.rs b/src/bibliography/bib_types/article.rs
index f9f0b83..9eb4d91 100644
--- a/src/bibliography/bib_types/article.rs
+++ b/src/bibliography/bib_types/article.rs
@@ -36,11 +36,11 @@ impl Article {
 }
 
 impl FromHashMap for Article {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let author = map.get(K_AUTHOR)?;
-        let title = map.get(K_TITLE)?;
-        let journal = map.get(K_JOURNAL)?;
-        let date = parse_date(map.get(K_DATE)?)?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let author = map.get(K_AUTHOR).ok_or(missing_field!(K_AUTHOR))?;
+        let title = map.get(K_TITLE).ok_or(missing_field!(K_TITLE))?;
+        let journal = map.get(K_JOURNAL).ok_or(missing_field!(K_JOURNAL))?;
+        let date = parse_date(map.get(K_DATE).ok_or(missing_field!(K_DATE))?)?;
         let mut article = Self::new(author.clone(), title.clone(), journal.clone(), date);
 
         article.volume = map.get(K_VOLUME).cloned();
@@ -49,6 +49,6 @@ impl FromHashMap for Article {
         article.pages = map.get(K_PAGES).cloned();
         article.url = map.get(K_URL).cloned();
 
-        Some(Box::new(article))
+        Ok(Box::new(article))
     }
 }
diff --git a/src/bibliography/bib_types/book.rs b/src/bibliography/bib_types/book.rs
index 5eb9f4d..65507af 100644
--- a/src/bibliography/bib_types/book.rs
+++ b/src/bibliography/bib_types/book.rs
@@ -37,11 +37,11 @@ impl Book {
 }
 
 impl FromHashMap for Book {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let author = map.get(K_AUTHOR)?;
-        let title = map.get(K_TITLE)?;
-        let publisher = map.get(K_PUBLISHER)?;
-        let date = parse_date(map.get(K_DATE)?)?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let author = map.get(K_AUTHOR).ok_or(missing_field!(K_AUTHOR))?;
+        let title = map.get(K_TITLE).ok_or(missing_field!(K_TITLE))?;
+        let publisher = map.get(K_PUBLISHER).ok_or(missing_field!(K_PUBLISHER))?;
+        let date = parse_date(map.get(K_DATE).ok_or(missing_field!(K_DATE))?)?;
         let mut book = Book::new(author.clone(), title.clone(), publisher.clone(), date);
 
         book.volume = map.get(K_VOLUME).cloned();
@@ -50,6 +50,6 @@ impl FromHashMap for Book {
         book.edition = map.get(K_EDITION).cloned();
         book.url = map.get(K_URL).cloned();
 
-        Some(Box::new(book))
+        Ok(Box::new(book))
     }
 }
diff --git a/src/bibliography/bib_types/booklet.rs b/src/bibliography/bib_types/booklet.rs
index 0db547e..f6c8068 100644
--- a/src/bibliography/bib_types/booklet.rs
+++ b/src/bibliography/bib_types/booklet.rs
@@ -28,15 +28,19 @@ impl Booklet {
 }
 
 impl FromHashMap for Booklet {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let title = map.get(K_TITLE)?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let title = map.get(K_TITLE).ok_or(missing_field!(K_TITLE))?;
         let mut booklet = Booklet::new(title.clone());
 
         booklet.author = map.get(K_AUTHOR).cloned();
         booklet.how_published = map.get(K_HOW_PUBLISHED).cloned();
         booklet.address = map.get(K_ADDRESS).cloned();
-        booklet.date = map.get(K_DATE).and_then(|d| parse_date(d));
+        booklet.date = map
+            .get(K_DATE)
+            .ok_or(missing_field!(K_DATE))
+            .and_then(|d| parse_date(d))
+            .ok();
 
-        Some(Box::new(booklet))
+        Ok(Box::new(booklet))
     }
 }
diff --git a/src/bibliography/bib_types/in_book.rs b/src/bibliography/bib_types/in_book.rs
index 1bf2614..75ea3de 100644
--- a/src/bibliography/bib_types/in_book.rs
+++ b/src/bibliography/bib_types/in_book.rs
@@ -1,5 +1,5 @@
 use crate::bibliography::keys::{
-    K_ADDRESS, K_AUTHOR, K_DATE, K_EDITION, K_PUBLISHER, K_SERIES, K_TITLE, K_VOLUME,
+    K_ADDRESS, K_AUTHOR, K_DATE, K_EDITION, K_POSITION, K_PUBLISHER, K_SERIES, K_TITLE, K_VOLUME,
 };
 use crate::bibliography::FromHashMap;
 use crate::utils::date::{parse_date, LocalDate};
@@ -44,12 +44,12 @@ impl InBook {
 }
 
 impl FromHashMap for InBook {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let author = map.get(K_AUTHOR)?;
-        let title = map.get(K_TITLE)?;
-        let position = map.get(K_TITLE)?;
-        let publisher = map.get(K_PUBLISHER)?;
-        let date = parse_date(map.get(K_DATE)?)?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let author = map.get(K_AUTHOR).ok_or(missing_field!(K_AUTHOR))?;
+        let title = map.get(K_TITLE).ok_or(missing_field!(K_TITLE))?;
+        let position = map.get(K_POSITION).ok_or(missing_field!(K_POSITION))?;
+        let publisher = map.get(K_PUBLISHER).ok_or(missing_field!(K_PUBLISHER))?;
+        let date = parse_date(map.get(K_DATE).ok_or(missing_field!(K_DATE))?)?;
         let mut in_book = InBook::new(
             author.clone(),
             title.clone(),
@@ -63,6 +63,6 @@ impl FromHashMap for InBook {
         in_book.address = map.get(K_ADDRESS).cloned();
         in_book.edition = map.get(K_EDITION).cloned();
 
-        Some(Box::new(in_book))
+        Ok(Box::new(in_book))
     }
 }
diff --git a/src/bibliography/bib_types/in_collection.rs b/src/bibliography/bib_types/in_collection.rs
index 940f2d3..1b6d215 100644
--- a/src/bibliography/bib_types/in_collection.rs
+++ b/src/bibliography/bib_types/in_collection.rs
@@ -41,28 +41,20 @@ impl InCollection {
 }
 
 impl FromHashMap for InCollection {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let author = map.get(K_AUTHOR)?;
-        let title = map.get(K_TITLE)?;
-        let publisher = map.get(K_PUBLISHER)?;
-        let date = parse_date(map.get(K_DATE)?)?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let author = map.get(K_AUTHOR).ok_or(missing_field!(K_AUTHOR))?;
+        let title = map.get(K_TITLE).ok_or(missing_field!(K_TITLE))?;
+        let publisher = map.get(K_PUBLISHER).ok_or(missing_field!(K_PUBLISHER))?;
+        let date = parse_date(map.get(K_DATE).ok_or(missing_field!(K_DATE))?)?;
         let mut in_col = InCollection::new(author.clone(), title.clone(), publisher.clone(), date);
 
         in_col.editor = map.get(K_EDITOR).cloned();
         in_col.volume = map.get(K_VOLUME).cloned();
-        if let Some(series) = map.get(K_SERIES) {
-            Some(series.clone());
-        }
-        if let Some(position) = map.get(K_POSITION) {
-            in_col.position = Some(position.clone());
-        }
-        if let Some(address) = map.get(K_ADDRESS) {
-            in_col.address = Some(address.clone());
-        }
-        if let Some(edition) = map.get(K_EDITION) {
-            in_col.edition = Some(edition.clone());
-        }
+        in_col.series = map.get(K_SERIES).cloned();
+        in_col.position = map.get(K_POSITION).cloned();
+        in_col.address = map.get(K_ADDRESS).cloned();
+        in_col.edition = map.get(K_EDITION).cloned();
 
-        Some(Box::new(in_col))
+        Ok(Box::new(in_col))
     }
 }
diff --git a/src/bibliography/bib_types/manual.rs b/src/bibliography/bib_types/manual.rs
index b923314..7061108 100644
--- a/src/bibliography/bib_types/manual.rs
+++ b/src/bibliography/bib_types/manual.rs
@@ -30,16 +30,20 @@ impl Manual {
 }
 
 impl FromHashMap for Manual {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let title = map.get(K_TITLE)?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let title = map.get(K_TITLE).ok_or(missing_field!(K_TITLE))?;
         let mut manual = Manual::new(title.clone());
 
         manual.author = map.get(K_AUTHOR).cloned();
         manual.organization = map.get(K_ORGANIZATION).cloned();
         manual.address = map.get(K_ADDRESS).cloned();
         manual.edition = map.get(K_EDITION).cloned();
-        manual.date = map.get(K_DATE).and_then(|d| parse_date(d));
+        manual.date = map
+            .get(K_DATE)
+            .ok_or(missing_field!(K_DATE))
+            .and_then(|d| parse_date(d))
+            .ok();
 
-        Some(Box::new(manual))
+        Ok(Box::new(manual))
     }
 }
diff --git a/src/bibliography/bib_types/misc.rs b/src/bibliography/bib_types/misc.rs
index 53768b9..bb810e0 100644
--- a/src/bibliography/bib_types/misc.rs
+++ b/src/bibliography/bib_types/misc.rs
@@ -28,15 +28,19 @@ impl Misc {
 }
 
 impl FromHashMap for Misc {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
         let mut misc = Misc::new();
 
         misc.author = map.get(K_AUTHOR).cloned();
         misc.title = map.get(K_TITLE).cloned();
         misc.url = map.get(K_URL).cloned();
         misc.how_published = map.get(K_HOW_PUBLISHED).cloned();
-        misc.date = map.get(K_DATE).and_then(|d| parse_date(d));
+        misc.date = map
+            .get(K_DATE)
+            .ok_or(missing_field!(K_DATE))
+            .and_then(|d| parse_date(d))
+            .ok();
 
-        Some(Box::new(misc))
+        Ok(Box::new(misc))
     }
 }
diff --git a/src/bibliography/bib_types/mod.rs b/src/bibliography/bib_types/mod.rs
index c3458e2..91f2101 100644
--- a/src/bibliography/bib_types/mod.rs
+++ b/src/bibliography/bib_types/mod.rs
@@ -68,25 +68,25 @@ impl BibliographyType {
 }
 
 impl FromHashMap for BibliographyType {
-    fn from_hash_map(map: &HashMap<String, String>) -> Option<Box<Self>> {
-        match map.get(K_TYPE)?.as_str() {
-            T_ARTICLE => Some(Box::new(Self::Article(*Article::from_hash_map(map)?))),
-            T_BOOK => Some(Box::new(Self::Book(*Book::from_hash_map(map)?))),
-            T_BOOKLET => Some(Box::new(Self::Booklet(*Booklet::from_hash_map(map)?))),
-            T_IN_BOOK => Some(Box::new(Self::InBook(*InBook::from_hash_map(map)?))),
-            T_IN_COLLECTION => Some(Box::new(Self::InCollection(*InCollection::from_hash_map(
+    fn from_hash_map(map: &HashMap<String, String>) -> Result<Box<Self>, String> {
+        match map.get(K_TYPE).ok_or(missing_field!(K_TYPE))?.as_str() {
+            T_ARTICLE => Ok(Box::new(Self::Article(*Article::from_hash_map(map)?))),
+            T_BOOK => Ok(Box::new(Self::Book(*Book::from_hash_map(map)?))),
+            T_BOOKLET => Ok(Box::new(Self::Booklet(*Booklet::from_hash_map(map)?))),
+            T_IN_BOOK => Ok(Box::new(Self::InBook(*InBook::from_hash_map(map)?))),
+            T_IN_COLLECTION => Ok(Box::new(Self::InCollection(*InCollection::from_hash_map(
                 map,
             )?))),
-            T_MANUAL => Some(Box::new(Self::Manual(*Manual::from_hash_map(map)?))),
-            T_MISC => Some(Box::new(Self::Misc(*Misc::from_hash_map(map)?))),
-            T_REPOSITORY => Some(Box::new(Self::Repository(*Repository::from_hash_map(map)?))),
-            T_TECH_REPORT => Some(Box::new(Self::TechReport(*TechReport::from_hash_map(map)?))),
-            T_THESIS => Some(Box::new(Self::Thesis(*Thesis::from_hash_map(map)?))),
-            T_UNPUBLISHED => Some(Box::new(Self::Unpublished(*Unpublished::from_hash_map(
+            T_MANUAL => Ok(Box::new(Self::Manual(*Manual::from_hash_map(map)?))),
+            T_MISC => Ok(Box::new(Self::Misc(*Misc::from_hash_map(map)?))),
+            T_REPOSITORY => Ok(Box::new(Self::Repository(*Repository::from_hash_map(map)?))),
+            T_TECH_REPORT => Ok(Box::new(Self::TechReport(*TechReport::from_hash_map(map)?))),
+            T_THESIS => Ok(Box::new(Self::Thesis(*Thesis::from_hash_map(map)?))),
+            T_UNPUBLISHED => Ok(Box::new(Self::Unpublished(*Unpublished::from_hash_map(
                 map,
             )?))),
-            T_WEBSITE => Some(Box::new(Self::Website(*Website::from_hash_map(map)?))),
-            _ => None,
+            T_WEBSITE => Ok(Box::new(Self::Website(*Website::from_hash_map(map)?))),
+            _ => Err(format!("Unknown type")),
         }
     }
 }
diff --git a/src/bibliography/bib_types/repository.rs b/src/bibliography/bib_types/repository.rs
index 91d72b8..29c0503 100644
--- a/src/bibliography/bib_types/repository.rs
+++ b/src/bibliography/bib_types/repository.rs
@@ -31,16 +31,20 @@ impl Repository {
 }
 
 impl FromHashMap for Repository {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let author = map.get(K_AUTHOR)?;
-        let title = map.get(K_TITLE)?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let author = map.get(K_AUTHOR).ok_or(missing_field!(K_AUTHOR))?;
+        let title = map.get(K_TITLE).ok_or(missing_field!(K_TITLE))?;
         let mut repo = Repository::new(author.clone(), title.clone());
 
         repo.url = map.get(K_URL).cloned();
         repo.license = map.get(K_LICENSE).cloned();
         repo.cms = map.get(K_CMS).cloned();
-        repo.accessed_at = map.get(K_ACCESSED_AT).and_then(|d| parse_date(d));
+        repo.accessed_at = map
+            .get(K_ACCESSED_AT)
+            .ok_or(missing_field!(K_ACCESSED_AT))
+            .and_then(|d| parse_date(d))
+            .ok();
 
-        Some(Box::new(repo))
+        Ok(Box::new(repo))
     }
 }
diff --git a/src/bibliography/bib_types/tech_report.rs b/src/bibliography/bib_types/tech_report.rs
index 77bd6a0..c0fbcbf 100644
--- a/src/bibliography/bib_types/tech_report.rs
+++ b/src/bibliography/bib_types/tech_report.rs
@@ -30,17 +30,22 @@ impl TechReport {
 }
 
 impl FromHashMap for TechReport {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let author = map.get(K_AUTHOR)?;
-        let title = map.get(K_TITLE)?;
-        let institution = map.get(K_INSTITUTION)?;
-        let date = map.get(K_DATE).and_then(|d| parse_date(d))?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let author = map.get(K_AUTHOR).ok_or(missing_field!(K_AUTHOR))?;
+        let title = map.get(K_TITLE).ok_or(missing_field!(K_TITLE))?;
+        let institution = map
+            .get(K_INSTITUTION)
+            .ok_or(missing_field!(K_INSTITUTION))?;
+        let date = map
+            .get(K_DATE)
+            .ok_or(missing_field!(K_DATE))
+            .and_then(|d| parse_date(d))?;
         let mut tech_report =
             TechReport::new(author.clone(), title.clone(), institution.clone(), date);
 
         tech_report.number = map.get(K_NUMBER).cloned();
         tech_report.address = map.get(K_ADDRESS).cloned();
 
-        Some(Box::new(tech_report))
+        Ok(Box::new(tech_report))
     }
 }
diff --git a/src/bibliography/bib_types/thesis.rs b/src/bibliography/bib_types/thesis.rs
index 8b82568..bc7a758 100644
--- a/src/bibliography/bib_types/thesis.rs
+++ b/src/bibliography/bib_types/thesis.rs
@@ -28,15 +28,18 @@ impl Thesis {
 }
 
 impl FromHashMap for Thesis {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let author = map.get(K_AUTHOR)?;
-        let title = map.get(K_TITLE)?;
-        let school = map.get(K_SCHOOL)?;
-        let date = map.get(K_DATE).and_then(|d| parse_date(d))?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let author = map.get(K_AUTHOR).ok_or(missing_field!(K_AUTHOR))?;
+        let title = map.get(K_TITLE).ok_or(missing_field!(K_TITLE))?;
+        let school = map.get(K_SCHOOL).ok_or(missing_field!(K_SCHOOL))?;
+        let date = map
+            .get(K_DATE)
+            .ok_or(missing_field!(K_DATE))
+            .and_then(|d| parse_date(d))?;
         let mut thesis = Thesis::new(author.clone(), title.clone(), school.clone(), date.clone());
 
         thesis.address = map.get(K_ADDRESS).cloned();
 
-        Some(Box::new(thesis))
+        Ok(Box::new(thesis))
     }
 }
diff --git a/src/bibliography/bib_types/unpublished.rs b/src/bibliography/bib_types/unpublished.rs
index 2b5691c..aeecd83 100644
--- a/src/bibliography/bib_types/unpublished.rs
+++ b/src/bibliography/bib_types/unpublished.rs
@@ -24,13 +24,13 @@ impl Unpublished {
 }
 
 impl FromHashMap for Unpublished {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let author = map.get(K_AUTHOR)?;
-        let title = map.get(K_TITLE)?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let author = map.get(K_AUTHOR).ok_or(missing_field!(K_AUTHOR))?;
+        let title = map.get(K_TITLE).ok_or(missing_field!(K_TITLE))?;
         let mut unpub = Unpublished::new(author.clone(), title.clone());
 
-        unpub.date = map.get(K_DATE).and_then(|d| parse_date(d));
+        unpub.date = map.get(K_DATE).and_then(|d| parse_date(d).ok());
 
-        Some(Box::new(unpub))
+        Ok(Box::new(unpub))
     }
 }
diff --git a/src/bibliography/bib_types/website.rs b/src/bibliography/bib_types/website.rs
index 1c7f5ea..604900f 100644
--- a/src/bibliography/bib_types/website.rs
+++ b/src/bibliography/bib_types/website.rs
@@ -28,15 +28,15 @@ impl Website {
 }
 
 impl FromHashMap for Website {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let url = map.get(K_URL)?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let url = map.get(K_URL).ok_or(missing_field!(K_URL))?;
         let mut website = Website::new(url.clone());
 
         website.title = map.get(K_TITLE).cloned();
         website.author = map.get(K_AUTHOR).cloned();
-        website.accessed_at = map.get(K_ACCESSED_AT).and_then(|d| parse_date(d));
-        website.date = map.get(K_DATE).and_then(|d| parse_date(d));
+        website.accessed_at = map.get(K_ACCESSED_AT).and_then(|d| parse_date(d).ok());
+        website.date = map.get(K_DATE).and_then(|d| parse_date(d).ok());
 
-        Some(Box::new(website))
+        Ok(Box::new(website))
     }
 }
diff --git a/src/bibliography/bibliography_dict.rs b/src/bibliography/bibliography_dict.rs
index e0d789f..70c57c9 100644
--- a/src/bibliography/bibliography_dict.rs
+++ b/src/bibliography/bibliography_dict.rs
@@ -25,13 +25,13 @@ impl BibliographyDictionary {
     }
 
     /// Inserts a bibliography entry represented as a HashMap
-    pub fn insert_map(&mut self, map: &HashMap<String, String>) -> Option<()> {
-        let key = map.get(K_KEY)?;
+    pub fn insert_map(&mut self, map: &HashMap<String, String>) -> Result<(), String> {
+        let key = map.get(K_KEY).ok_or(missing_field!(K_KEY))?;
         let entry = *BibliographyEntry::from_hash_map(map)?;
         self.entries
             .insert(key.clone(), Arc::new(Mutex::new(entry)));
 
-        Some(())
+        Ok(())
     }
 
     /// Returns the reference to the bibliography entry with the given key
diff --git a/src/bibliography/bibliography_entry.rs b/src/bibliography/bibliography_entry.rs
index 39d9bab..53d807c 100644
--- a/src/bibliography/bibliography_entry.rs
+++ b/src/bibliography/bibliography_entry.rs
@@ -35,8 +35,8 @@ impl BibliographyEntry {
 }
 
 impl FromHashMap for BibliographyEntry {
-    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Option<Box<Self>> {
-        let key = map.get(K_KEY)?;
+    fn from_hash_map(map: &HashMap<String, String, RandomState>) -> Result<Box<Self>, String> {
+        let key = map.get(K_KEY).ok_or(missing_field!(K_KEY))?;
         let bib_type = BibliographyType::from_hash_map(map)?;
 
         let mut entry = Self::new(key.clone());
@@ -45,6 +45,6 @@ impl FromHashMap for BibliographyEntry {
         entry.bib_type = *bib_type;
         entry.raw_fields = map.clone();
 
-        Some(Box::new(entry))
+        Ok(Box::new(entry))
     }
 }
diff --git a/src/bibliography/mod.rs b/src/bibliography/mod.rs
index f343527..917eda9 100644
--- a/src/bibliography/mod.rs
+++ b/src/bibliography/mod.rs
@@ -1,5 +1,11 @@
 use std::collections::HashMap;
 
+macro_rules! missing_field {
+    ($e:expr) => {
+        format!("Missing field '{}'", $e)
+    };
+}
+
 pub mod bib_types;
 pub mod bibliography_dict;
 pub mod bibliography_entry;
@@ -8,5 +14,5 @@ pub mod keys;
 /// A trait that provides the from_has_map function that can be used
 /// to create a bibliography source type from a hashmap
 pub trait FromHashMap {
-    fn from_hash_map(map: &HashMap<String, String>) -> Option<Box<Self>>;
+    fn from_hash_map(map: &HashMap<String, String>) -> Result<Box<Self>, String>;
 }
diff --git a/src/lib.rs b/src/lib.rs
index 25dd4e5..182b758 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -50,7 +50,12 @@ mod tests {
         map.insert("title".to_string(), "test_title".to_string());
         map.insert("journal".to_string(), "test_journal".to_string());
         map.insert("date".to_string(), "01.09.2020".to_string());
-        manager.entry_dictionary().lock().unwrap().insert_map(&map);
+        manager
+            .entry_dictionary()
+            .lock()
+            .unwrap()
+            .insert_map(&map)
+            .unwrap();
         manager.assign_entries_to_references();
 
         assert!(anchor1.lock().unwrap().entry.is_some());
diff --git a/src/utils/date.rs b/src/utils/date.rs
index 57dddbf..ff6de31 100644
--- a/src/utils/date.rs
+++ b/src/utils/date.rs
@@ -3,11 +3,10 @@ use chrono_english::Dialect;
 
 pub type LocalDate = Date<Local>;
 
-pub fn parse_date(date_str: &str) -> Option<LocalDate> {
+pub fn parse_date(date_str: &str) -> Result<LocalDate, String> {
     let date_str = date_str.replace('.', "/");
-    if let Ok(datetime) = chrono_english::parse_date_string(&date_str, Local::now(), Dialect::Us) {
-        Some(datetime.date())
-    } else {
-        None
-    }
+
+    chrono_english::parse_date_string(&date_str, Local::now(), Dialect::Us)
+        .map(|d| d.date())
+        .map_err(|e| e.to_string())
 }