diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -397,13 +397,13 @@
             self._names = []
         self._rbcnamescount = len(self._names) # number of names read at
                                                # _rbcsnameslen
-        self._namesreverse = dict((b, r) for r, b in enumerate(self._names))
+        self._namesreverse = None
 
     def _clear(self):
         self._rbcsnameslen = 0
         del self._names[:]
         self._rbcnamescount = 0
-        self._namesreverse.clear()
+        self._namesreverse = None
         self._rbcrevslen = len(self._repo.changelog)
         self._rbcrevs = bytearray(self._rbcrevslen * _rbcrecsize)
 
@@ -453,6 +453,8 @@
         """Retrieve branch info from changelog and update _rbcrevs"""
         changelog = self._repo.changelog
         b, close = changelog.branchinfo(rev)
+        if self._namesreverse is None:
+            self._namesreverse = dict((b, r) for r, b in enumerate(self._names))
         if b in self._namesreverse:
             branchidx = self._namesreverse[b]
         else:
@@ -467,6 +469,8 @@
 
     def setdata(self, branch, rev, node, close):
         """add new data information to the cache"""
+        if self._namesreverse is None:
+            self._namesreverse = dict((b, r) for r, b in enumerate(self._names))
         if branch in self._namesreverse:
             branchidx = self._namesreverse[branch]
         else: