diff -r e45de0f47215 -r bb441d77df99 mercurial/filemerge.py
--- a/mercurial/filemerge.py	Sun Feb 03 23:08:16 2008 -0800
+++ b/mercurial/filemerge.py	Mon Feb 04 09:51:56 2008 -0600
@@ -9,7 +9,7 @@
 from i18n import _
 import util, os, tempfile, context, simplemerge, re
 
-def _toolstr(ui, tool, part, default=None):
+def _toolstr(ui, tool, part, default=""):
     return ui.config("merge-tools", tool + "." + part, default)
 
 def _toolbool(ui, tool, part, default=False):