diff --git a/mercurial/encoding.py b/mercurial/encoding.py
--- a/mercurial/encoding.py
+++ b/mercurial/encoding.py
@@ -190,6 +190,11 @@ def lower(s):
 def upper(s):
     "best-effort encoding-aware case-folding of local string s"
     try:
+        s.decode('ascii') # throw exception for non-ASCII character
+        return s.upper()
+    except UnicodeDecodeError:
+        pass
+    try:
         if isinstance(s, localstr):
             u = s._utf8.decode("utf-8")
         else: