diff --git a/faust/assignor/client_assignment.py b/faust/assignor/client_assignment.py
index 3b1205f23..218ed528f 100644
--- a/faust/assignor/client_assignment.py
+++ b/faust/assignor/client_assignment.py
@@ -149,7 +149,7 @@ class ClientMetadata(
     assignment: ClientAssignment
     url: str
     changelog_distribution: HostToPartitionMap
-    external_topic_distribution: HostToPartitionMap
+    external_topic_distribution: HostToPartitionMap = cast(HostToPartitionMap, {})
     topic_groups: Mapping[str, int] = cast(Mapping[str, int], None)
 
     def __post_init__(self) -> None: