diff --git test/mitmproxy/addons/test_readfile.py test/mitmproxy/addons/test_readfile.py
index 3d28d8b7d..94e18cdb3 100644
--- test/mitmproxy/addons/test_readfile.py
+++ test/mitmproxy/addons/test_readfile.py
@@ -30,8 +30,8 @@ def data():
 
 
 @pytest.fixture
-def corrupt_data():
-    f = data()
+def corrupt_data(data):
+    f = io.BytesIO(data.getvalue())
     f.seek(0, io.SEEK_END)
     f.write(b"qibble")
     f.seek(0)