diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -341,6 +341,10 @@ class revbranchcache(object):
         changelog = self._repo.changelog
         rbcrevidx = rev * _rbcrecsize
 
+        # avoid negative index, changelog.read(nullrev) is fast without cache
+        if rev == nullrev:
+            return changelog.branchinfo(rev)
+
         # if requested rev is missing, add and populate all missing revs
         if len(self._rbcrevs) < rbcrevidx + _rbcrecsize:
             self._rbcrevs.extend('\0' * (len(changelog) * _rbcrecsize -
diff --git a/tests/test-revset.t b/tests/test-revset.t
--- a/tests/test-revset.t
+++ b/tests/test-revset.t
@@ -1602,6 +1602,14 @@ prepare repository that has "default" br
   $ echo default5 >> a
   $ hg ci -m5
 
+"null" revision belongs to "default" branch (issue4683)
+
+  $ log 'branch(null)'
+  0
+  1
+  4
+  5
+
 "null" revision belongs to "default" branch, but it shouldn't appear in set
 unless explicitly specified (issue4682)