--- a/mercurial/discovery.py	Mon Sep 30 17:42:38 2013 +0200
+++ b/mercurial/discovery.py	Tue Oct 01 00:26:22 2013 +0900
@@ -269,13 +269,12 @@
     allfuturecommon = set(c.node() for c in repo.set('%ld', outgoing.common))
     allfuturecommon.update(allmissing)
     for branch, heads in sorted(headssum.iteritems()):
-        if heads[0] is None:
-            # Maybe we should abort if we push more that one head
-            # for new branches ?
-            continue
         candidate_newhs = set(heads[1])
         # add unsynced data
-        oldhs = set(heads[0])
+        if heads[0] is None:
+            oldhs = set()
+        else:
+            oldhs = set(heads[0])
         oldhs.update(heads[2])
         candidate_newhs.update(heads[2])
         dhs = None
@@ -310,7 +309,16 @@
             newhs = candidate_newhs
         if [h for h in heads[2] if h not in discardedheads]:
             unsynced = True
-        if len(newhs) > len(oldhs):
+        if heads[0] is None:
+            if 1 < len(newhs):
+                dhs = list(newhs)
+                if error is None:
+                    error = (_("push creates multiple headed new branch '%s'")
+                             % (branch))
+                    hint = _("merge or"
+                             " see \"hg help push\" for detail about"
+                             " pushing new heads")
+        elif len(newhs) > len(oldhs):
             # strip updates to existing remote heads from the new heads list
             dhs = sorted(newhs - bookmarkedheads - oldhs)
         if dhs:

--- a/tests/test-push-warn.t	Mon Sep 30 17:42:38 2013 +0200
+++ b/tests/test-push-warn.t	Tue Oct 01 00:26:22 2013 +0900
@@ -354,6 +354,29 @@
   adding file changes
   added 1 changesets with 1 changes to 1 files
 
+Pushing muliple headed new branch:
+
+  $ echo 14 > foo
+  $ hg -q branch f
+  $ hg -q ci -m 14
+  $ echo 15 > foo
+  $ hg -q ci -m 15
+  $ hg -q up 14
+  $ echo 16 > foo
+  $ hg -q ci -m 16
+  $ hg push --branch f --new-branch ../f
+  pushing to ../f
+  searching for changes
+  abort: push creates multiple headed new branch 'f'
+  (merge or see "hg help push" for detail about pushing new heads)
+  [255]
+  $ hg push --branch f --new-branch --force ../f
+  pushing to ../f
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 3 changesets with 3 changes to 1 files (+1 heads)
 
 Checking prepush logic does not allow silently pushing
 multiple new heads: