--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -441,6 +441,9 @@ def update(repo, node, branchmerge, forc
     wlock = working dir lock, if already held
     """
 
+    if node is None:
+        node = "tip"
+
     if not wlock:
         wlock = repo.wlock()