diff --git a/Lib/ntpath.py b/Lib/ntpath.py
index 8d50294..7972aa9 100644
--- a/Lib/ntpath.py
+++ b/Lib/ntpath.py
@@ -78,6 +78,9 @@
     """Normalize case of pathname.
 
     Makes all characters lowercase and all slashes into backslashes."""
+    if not isinstance(s, (bytes, str)):
+        raise TypeError("normcase() argument must be str or bytes, "
+                        "not '{}'".format(s.__class__.__name__))
     return s.replace(_get_altsep(s), _get_sep(s)).lower()