diff --git a/conservancy_beancount/beancount_types.py b/conservancy_beancount/beancount_types.py
index e89197b..56956ec 100644
--- a/conservancy_beancount/beancount_types.py
+++ b/conservancy_beancount/beancount_types.py
@@ -46,8 +46,8 @@ class Transaction(Directive):
     flag: bc_data.Flag
     payee: Optional[str]
     narration: str
-    tags: Set
-    links: Set
+    tags: Set[str]
+    links: Set[str]
     postings: List[Posting]
 
 
diff --git a/conservancy_beancount/errors.py b/conservancy_beancount/errors.py
index 486676a..62891c5 100644
--- a/conservancy_beancount/errors.py
+++ b/conservancy_beancount/errors.py
@@ -118,7 +118,7 @@ class InvalidMetadataError(Error):
                  key: MetaKey,
                  value: Optional[MetaValue]=None,
                  post: Optional[bc_data.Posting]=None,
-                 need_type: Type=str,
+                 need_type: Type[object]=str,
                  source: Meta=None,
     ) -> None:
         if post is None: