diff -r 833be17000b6 -r 65f1b97484be mercurial/filemerge.py
--- a/mercurial/filemerge.py	Fri Apr 11 15:14:00 2008 +0200
+++ b/mercurial/filemerge.py	Sun Apr 13 11:31:45 2008 -0500
@@ -16,6 +16,8 @@
     return ui.configbool("merge-tools", tool + "." + part, default)
 
 def _findtool(ui, tool):
+    if tool in ("internal:fail", "internal:local", "internal:other"):
+        return tool
     k = _toolstr(ui, tool, "regkey")
     if k:
         p = util.lookup_reg(k, _toolstr(ui, tool, "regname"))