diff -r 25d78aa1ec21 Lib/locale.py
--- a/Lib/locale.py	Wed May 13 21:44:02 2015 -0700
+++ b/Lib/locale.py	Fri May 15 11:37:16 2015 +0200
@@ -484,6 +484,10 @@
         return tuple(code.split('.')[:2])
     elif code == 'C':
         return None, None
+    elif sys.platform == 'darwin' and code == 'UTF-8':
+        # On MacOSX "LC_CTYPE=UTF-8" is a valid locale setting
+        # for getting UTF-8 handling for text.
+        return None, 'UTF-8'
     raise ValueError('unknown locale: %s' % localename)
 
 def _build_localename(localetuple):