diff --git a/conservancy_beancount/reports/core.py b/conservancy_beancount/reports/core.py
index 25a872b3b7e0ea90e70d2cb276196e8c17b8152c..2ef93c8ac532526e73336339cc015308a6b776e5 100644
--- a/conservancy_beancount/reports/core.py
+++ b/conservancy_beancount/reports/core.py
@@ -106,7 +106,7 @@ class RelatedPostings(Sequence[data.Posting]):
                       postings: Iterable[data.Posting],
                       key: MetaKey,
                       default: Optional[MetaValue]=None,
-    ) -> Mapping[MetaKey, 'RelatedPostings']:
+    ) -> Mapping[Optional[MetaValue], 'RelatedPostings']:
         """Relate postings by metadata value
 
         This method takes an iterable of postings and returns a mapping.
@@ -114,7 +114,7 @@ class RelatedPostings(Sequence[data.Posting]):
         The values are RelatedPostings instances that contain all the postings
         that had that same metadata value.
         """
-        retval: DefaultDict[MetaKey, 'RelatedPostings'] = collections.defaultdict(cls)
+        retval: DefaultDict[Optional[MetaValue], 'RelatedPostings'] = collections.defaultdict(cls)
         for post in postings:
             retval[post.meta.get(key, default)].add(post)
         retval.default_factory = None