diff --git a/testing/test_assertrewrite.py b/testing/test_assertrewrite.py
index 2501ec16b..0e9d7c264 100644
--- a/testing/test_assertrewrite.py
+++ b/testing/test_assertrewrite.py
@@ -288,7 +288,7 @@ class TestRewriteOnImport:
         sub.join("test_readonly.py").write("""
 def test_rewritten():
     assert "@py_builtins" in globals()
-""")
+""", "wb")
         sub.chmod(320)
         assert testdir.runpytest().ret == 0