diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -696,6 +696,34 @@ def update(repo, node, branchmerge, forc
                     node = repo.lookup("tip") # update to tip
                 else:
                     raise util.Abort(_("branch %s not found") % wc.branch())
+
+            if p1.obsolete() and not p1.children():
+                # allow updating to successors
+                successors = obsolete.successorssets(repo, p1.node())
+
+                # behavior of certain cases is as follows,
+                #
+                # divergent changesets: update to highest rev, similar to what
+                #     is currently done when there are more than one head
+                #     (i.e. 'tip')
+                #
+                # replaced changesets: same as divergent except we know there
+                # is no conflict
+                #
+                # pruned changeset: no update is done; though, we could
+                #     consider updating to the first non-obsolete parent,
+                #     similar to what is current done for 'hg prune'
+
+                if successors:
+                    # flatten the list here handles both divergent (len > 1)
+                    # and the usual case (len = 1)
+                    successors = [n for sub in successors for n in sub]
+
+                    # get the max revision for the given successors set,
+                    # i.e. the 'tip' of a set
+                    node = repo.revs("max(%ln)", successors)[0]
+                    pa = p1
+
         overwrite = force and not branchmerge
 
         p2 = repo[node]
diff --git a/tests/test-update-branches.t b/tests/test-update-branches.t
--- a/tests/test-update-branches.t
+++ b/tests/test-update-branches.t
@@ -229,6 +229,14 @@ the bookmark (issue4015)
   $ hg bookmarks
    * bm                        5:ff252e8273df
 
+Test that 4 is detected as the no-argument destination from 3
+  $ hg up --quiet 0          # we should be able to update to 3 directly
+  $ hg up --quiet --hidden 3 # but not implemented yet.
+  $ hg up
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id
+  d047485b3896+ (b1)
+
 Test that 5 is detected as a valid destination from 1
   $ hg up --quiet 0          # we should be able to update to 3 directly
   $ hg up --quiet --hidden 3 # but not implemented yet.
@@ -242,3 +250,14 @@ Test that 5 is not detected as a valid d
   abort: uncommitted changes
   (commit or update --clean to discard changes)
   [255]
+
+Test that we don't crash when updating from a pruned changeset (i.e. has no
+successors). Behavior should probably be that we update to the first
+non-obsolete parent but that will be decided later.
+  $ hg id --debug -r 2
+  bd10386d478cd5a9faf2e604114c8e6da62d3889
+  $ hg up --quiet 0
+  $ hg up --quiet 2
+  $ hg debugobsolete bd10386d478cd5a9faf2e604114c8e6da62d3889
+  $ hg up
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved