--- a/hgext/acl.py	Mon May 10 21:15:12 2010 +0200
+++ b/hgext/acl.py	Mon May 10 21:44:00 2010 +0200
@@ -161,7 +161,10 @@
 
     ui.debug('acl: "%s" not defined in [acl.groups]\n' % group)
     # If no users found in group definition, get users from OS-level group
-    return util.groupmembers(group)
+    try:
+        return util.groupmembers(group)
+    except KeyError:
+        raise util.Abort(_("group '%s' is undefined") % group)
 
 def _usermatch(ui, user, usersorgroups):