--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -477,6 +477,7 @@ def update(repo, node, branchmerge, forc
         repo.dirstate.setparents(fp1, fp2)
         repo.hook('update', parent1=xp1, parent2=xp2, error=stats[3])
         if not branchmerge:
+            b = util.tolocal(p2.branch())
             repo.opener("branch", "w").write(p2.branch() + "\n")
 
     return stats