diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
index 2768e114794635..512e354fabc89f 100644
--- a/Lib/test/support/__init__.py
+++ b/Lib/test/support/__init__.py
@@ -852,7 +852,11 @@ def dec(*args, **kwargs):
     '\u20AC',
 ):
     try:
-        os.fsdecode(os.fsencode(character))
+        # If Python is set up to use the legacy 'mbcs' in Windows,
+        # 'replace' error mode is used, and encode() returns b'?'
+        # for characters missing in the ANSI codepage
+        if os.fsdecode(os.fsencode(character)) != character:
+            raise UnicodeError
     except UnicodeError:
         pass
     else: