diff --git a/mercurial/repo.py b/mercurial/repo.py
--- a/mercurial/repo.py
+++ b/mercurial/repo.py
@@ -9,6 +9,9 @@
 class RepoError(Exception):
     pass
 
+class LookupError(RepoError):
+    pass
+
 class repository(object):
     def capable(self, name):
         '''tell whether repo supports named capability.