diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -1467,7 +1467,7 @@
 
 def update(repo, node, branchmerge, force, ancestor=None,
            mergeancestor=False, labels=None, matcher=None, mergeforce=False,
-           updatecheck=None):
+           updatecheck=None, wc=None):
     """
     Perform a merge between the working directory and the given node
 
@@ -1538,7 +1538,8 @@
     else:
         partial = True
     with repo.wlock():
-        wc = repo[None]
+        if wc is None:
+            wc = repo[None]
         pl = wc.parents()
         p1 = pl[0]
         pas = [None]