diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -369,7 +369,7 @@ class localrepository(repo.repository):
             if not (lrev < self.changelog.count() and
                     self.changelog.node(lrev) == last): # sanity check
                 # invalidate the cache
-                raise ValueError('Invalid branch cache: unknown tip')
+                raise ValueError('invalidating branch cache (tip differs)')
             for l in lines:
                 if not l: continue
                 node, label = l.split(" ", 1)
diff --git a/tests/test-newbranch.out b/tests/test-newbranch.out
--- a/tests/test-newbranch.out
+++ b/tests/test-newbranch.out
@@ -63,7 +63,7 @@ user:        test
 date:        Mon Jan 12 13:46:40 1970 +0000
 summary:     modify a branch
 
-Invalid branch cache: unknown tip
+invalidating branch cache (tip differs)
 changeset:   4:4909a3732169c0c20011c4f4b8fdff4e3d89b23f
 branch:      foo
 tag:         tip