diff --git a/synapse/types/state.py b/synapse/types/state.py
index 937ffe3f979..3c61cea992e 100644
--- a/synapse/types/state.py
+++ b/synapse/types/state.py
@@ -591,6 +591,9 @@ def __contains__(self, key: Any) -> bool:
 
         typ, state_key = key
 
+        if not isinstance(typ, str) or not isinstance(state_key, str):
+            return False
+
         if typ in self.types:
             state_keys = self.types[typ]
             if state_keys is None or state_key in state_keys: