diff --git a/Lib/test/test_tabnanny.py b/Lib/test/test_tabnanny.py
index 95840d6ac0c5f4..7f3308d745aa09 100644
--- a/Lib/test/test_tabnanny.py
+++ b/Lib/test/test_tabnanny.py
@@ -292,8 +292,8 @@ def validate_cmd(self, *args, stdout="", stderr="", partial=False):
         _, out, err = script_helper.assert_python_ok('-m', 'tabnanny', *args)
         # Note: The `splitlines()` will solve the problem of CRLF(\r) added
         # by OS Windows.
-        out = out.decode('ascii')
-        err = err.decode('ascii')
+        out = os.fsdecode(out)
+        err = os.fsdecode(err)
         if partial:
             for std, output in ((stdout, out), (stderr, err)):
                 _output = output.splitlines()