diff --git a/conservancy_beancount/beancount_types.py b/conservancy_beancount/beancount_types.py
index c82abf59a536b1601ad80ba91537cc3e268c9425..960e52bd7c9fb41927fc052df385dcff7cee280d 100644
--- a/conservancy_beancount/beancount_types.py
+++ b/conservancy_beancount/beancount_types.py
@@ -49,7 +49,7 @@ class Directive(NamedTuple):
 
 
 class Error(NamedTuple):
-    source: Optional[Mapping[MetaKey, MetaValue]]
+    source: Mapping[MetaKey, MetaValue]
     message: str
     entry: Optional[Directive]