diff --git a/Lib/os2emxpath.py b/Lib/os2emxpath.py
index 184c9b1..0ccbf8a 100644
--- a/Lib/os2emxpath.py
+++ b/Lib/os2emxpath.py
@@ -36,6 +36,9 @@
     """Normalize case of pathname.
 
     Makes all characters lowercase and all altseps into seps."""
+    if not isinstance(s, (bytes, str)):
+        raise TypeError("normcase() argument must be str or bytes, "
+                        "not '{}'".format(s.__class__.__name__))
     return s.replace('\\', '/').lower()