diff --git a/Python/codecs.c b/Python/codecs.c
index 77eac8ef7d3..046abe35079 100644
--- a/Python/codecs.c
+++ b/Python/codecs.c
@@ -70,7 +70,7 @@ PyObject *normalizestring(const char *string)
         if (ch == ' ')
             ch = '-';
         else
-            ch = tolower(ch);
+            ch = tolower(Py_CHARMASK(ch));
 	p[i] = ch;
     }
     return v;