diff --git a/mercurial/posix.py b/mercurial/posix.py
index 3782be4dde..bbb2a2affa 100644
--- a/mercurial/posix.py
+++ b/mercurial/posix.py
@@ -194,6 +194,11 @@ if sys.platform == 'darwin':
     import fcntl # only needed on darwin, missing on jython
 
     def normcase(path):
+        try:
+            path.decode('ascii') # throw exception for non-ASCII character
+            return path.lower()
+        except UnicodeDecodeError:
+            pass
         try:
             u = path.decode('utf-8')
         except UnicodeDecodeError: