diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -9,13 +9,13 @@ from node import bin, hex, nullid, nullr
 import encoding
 
 def read(repo):
-    partial = {}
+    partial = branchcache()
     try:
         f = repo.opener("cache/branchheads")
         lines = f.read().split('\n')
         f.close()
     except (IOError, OSError):
-        return {}, nullid, nullrev
+        return branchcache(), nullid, nullrev
 
     try:
         last, lrev = lines.pop(0).split(" ", 1)
@@ -37,7 +37,7 @@ def read(repo):
     except Exception, inst:
         if repo.ui.debugflag:
             repo.ui.warn(str(inst), '\n')
-        partial, last, lrev = {}, nullid, nullrev
+        partial, last, lrev =  branchcache(), nullid, nullrev
     return partial, last, lrev
 
 def write(repo, branches, tip, tiprev):
@@ -143,3 +143,7 @@ def updatecache(repo):
         update(repo, partial, ctxgen)
     repo._branchcache = partial
     repo._branchcachetip = tip
+
+class branchcache(dict):
+    """A dict like object that hold branches heads cache"""
+
diff --git a/mercurial/discovery.py b/mercurial/discovery.py
--- a/mercurial/discovery.py
+++ b/mercurial/discovery.py
@@ -193,8 +193,9 @@ def _headssummary(repo, remote, outgoing
 
     # D. Update newmap with outgoing changes.
     # This will possibly add new heads and remove existing ones.
-    newmap = dict((branch, heads[1]) for branch, heads in headssum.iteritems()
-                  if heads[0] is not None)
+    newmap = branchmap.branchcache((branch, heads[1])
+                                 for branch, heads in headssum.iteritems()
+                                 if heads[0] is not None)
     branchmap.update(repo, newmap, missingctx)
     for branch, newheads in newmap.iteritems():
         headssum[branch][1][:] = newheads
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -666,7 +666,7 @@ class localrepository(object):
         '''returns a dictionary {branch: [branchheads]}'''
         if self.changelog.filteredrevs:
             # some changeset are excluded we can't use the cache
-            bmap = {}
+            bmap = branchmap.branchcache()
             branchmap.update(self, bmap, (self[r] for r in self))
             return bmap
         else:
@@ -2495,7 +2495,7 @@ class localrepository(object):
                 if rbheads:
                     rtiprev = max((int(self.changelog.rev(node))
                             for node in rbheads))
-                    self._branchcache = rbranchmap
+                    self._branchcache = branchmap.branchcache(rbranchmap)
                     rtipnode = self._branchcachetip = self[rtiprev].node()
                     branchmap.write(self, self._branchcache, rtipnode, rtiprev)
             self.invalidate()