--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -2563,7 +2563,10 @@ def _lookup(repo, node, branch=None):
             node = repo.lookup(node)
         else:
             wc = repo.workingctx()
-            node = repo.branchtags()[wc.branch()]
+            try:
+                node = repo.branchtags()[wc.branch()]
+            except KeyError:
+                raise util.Abort(_("branch %s not found") % wc.branch())
     return node
 
 def verify(ui, repo):

--- a/tests/test-newbranch
+++ b/tests/test-newbranch
@@ -46,3 +46,7 @@ hg id
 hg up -q 1
 hg up -q
 hg id
+hg branch foobar
+hg up
+
+exit 0

--- a/tests/test-newbranch.out
+++ b/tests/test-newbranch.out
@@ -81,3 +81,4 @@ 67ec16bde7f1575d523313b9bca000f6a6f12dca
 % update with no arguments: tipmost revision of the current branch
 bf1bc2f45e83
 4909a3732169 (foo) tip
+abort: branch foobar not found