diff --git a/src/replacers/date_replacer.py b/src/replacers/date_replacer.py
index 22bb7f3b53e4be83918b2f50b560786c1d85435a..de34f81cab481813c9ddc94818a88fcca816a04a 100644
--- a/src/replacers/date_replacer.py
+++ b/src/replacers/date_replacer.py
@@ -93,6 +93,8 @@ class DateReplacer(ReplacerInterface):
                             replacement.append(month_name)
                         elif entry[0] == DateDetection.AnnotationPart.OTHER:
                             replacement.append(entry[1])
+                        else:
+                            raise ValueError(f"Unknown format entry: {entry}")
 
                     replacement = "".join(replacement)
                     already_replaced[text[start:end]] = replacement