diff --git a/aioresponses/core.py b/aioresponses/core.py
index dc70961..2bb6d57 100644
--- a/aioresponses/core.py
+++ b/aioresponses/core.py
@@ -156,7 +156,7 @@ def _build_response(self, url: 'Union[URL, str]',
             method=method,
             headers=CIMultiDictProxy(CIMultiDict(**request_headers)),
         )
-        kwargs['writer'] = Mock()
+        kwargs['writer'] = None
         kwargs['continue100'] = None
         kwargs['timer'] = TimerNoop()
         kwargs['traces'] = []