diff --git a/interface_tester/interface_test.py b/interface_tester/interface_test.py
index 6959549..c597fac 100644
--- a/interface_tester/interface_test.py
+++ b/interface_tester/interface_test.py
@@ -430,7 +430,7 @@ def _cast_event(self, ctx: Context, raw_event: Union[str, _Event], relation: Rel
 
         if not isinstance(event, _Event):
             raise InvalidTestCaseError(
-                f"Bad interface test specification: event {raw_event} should be a relation event."
+                f"Bad interface test specification: event {raw_event} should be a relation event string or _Event."
             )
 
         # todo: if the user passes a relation event that is NOT about the relation