diff --git a/storey/sources.py b/storey/sources.py
index 4c6613d3..22f09b49 100644
--- a/storey/sources.py
+++ b/storey/sources.py
@@ -142,7 +142,7 @@ def _build_event(self, element, key):
         else:
             body = element
 
-        if hasattr(body, "__getitem__") and not key and self._key_field:
+        if isinstance(body, (dict, list)) and not key and self._key_field:
             if isinstance(self._key_field, str) or isinstance(self._key_field, int):
                 key = body[self._key_field]
             else:
diff --git a/tests/test_flow.py b/tests/test_flow.py
index 80f91c77..ffb277dc 100644
--- a/tests/test_flow.py
+++ b/tests/test_flow.py
@@ -3425,8 +3425,8 @@ def test_key_field_and_non_dict_event_body():
         ]
     ).run()
 
-    body1 = 1
-    body2 = 2
+    body1 = b"a"
+    body2 = b"b"
 
     controller.emit(body1)
     controller.emit(Event(body2, "my_key"))