diff --git a/swh/storage/in_memory.py b/swh/storage/in_memory.py
--- a/swh/storage/in_memory.py
+++ b/swh/storage/in_memory.py
@@ -629,7 +629,9 @@
         else:
             return None
 
-    def snapshot_count_branches(self, snapshot_id: Sha1Git) -> Optional[Dict[str, int]]:
+    def snapshot_count_branches(
+        self, snapshot_id: Sha1Git
+    ) -> Optional[Dict[Optional[str], int]]:
         snapshot = self._snapshots[snapshot_id]
         return collections.Counter(
             branch.target_type.value if branch else None
diff --git a/swh/storage/interface.py b/swh/storage/interface.py
--- a/swh/storage/interface.py
+++ b/swh/storage/interface.py
@@ -715,7 +715,9 @@
         ...
 
     @remote_api_endpoint("snapshot/count_branches")
-    def snapshot_count_branches(self, snapshot_id: Sha1Git) -> Optional[Dict[str, int]]:
+    def snapshot_count_branches(
+        self, snapshot_id: Sha1Git
+    ) -> Optional[Dict[Optional[str], int]]:
         """Count the number of branches in the snapshot with the given id
 
         Args:
diff --git a/swh/storage/storage.py b/swh/storage/storage.py
--- a/swh/storage/storage.py
+++ b/swh/storage/storage.py
@@ -771,7 +771,7 @@
     @db_transaction(statement_timeout=2000)
     def snapshot_count_branches(
         self, snapshot_id: Sha1Git, db=None, cur=None
-    ) -> Optional[Dict[str, int]]:
+    ) -> Optional[Dict[Optional[str], int]]:
         return dict([bc for bc in db.snapshot_count_branches(snapshot_id, cur)])
 
     @timed