diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -452,6 +452,8 @@ class revbranchcache(object):
 
     def _setcachedata(self, rev, node, branchidx):
         """Writes the node's branch data to the in-memory cache data."""
+        if rev == nullrev:
+            return
         rbcrevidx = rev * _rbcrecsize
         if len(self._rbcrevs) < rbcrevidx + _rbcrecsize:
             self._rbcrevs.extend('\0' *
diff --git a/tests/test-branches.t b/tests/test-branches.t
--- a/tests/test-branches.t
+++ b/tests/test-branches.t
@@ -1,5 +1,10 @@
   $ hg init a
   $ cd a
+
+Verify checking branch of nullrev before the cache is created doesnt crash
+  $ hg log -r 'branch(.)' -T '{branch}\n'
+
+Basic test
   $ echo 'root' >root
   $ hg add root
   $ hg commit -d '0 0' -m "Adding root node"