diff --git a/hgext/rebase.py b/hgext/rebase.py
--- a/hgext/rebase.py
+++ b/hgext/rebase.py
@@ -1316,9 +1316,9 @@ def pullrebase(orig, ui, repo, *args, **
                 ui.debug('--update and --rebase are not compatible, ignoring '
                          'the update flag\n')
 
-            ui.debug('before rebase: ensure working dir is clean\n')
             cmdutil.checkunfinished(repo)
-            cmdutil.bailifchanged(repo)
+            cmdutil.bailifchanged(repo, hint=_('cannot pull with rebase: '
+                'please commit or shelve your changes first'))
 
             revsprepull = len(repo)
             origpostincoming = commands.postincoming
diff --git a/mercurial/cmdutil.py b/mercurial/cmdutil.py
--- a/mercurial/cmdutil.py
+++ b/mercurial/cmdutil.py
@@ -355,15 +355,23 @@ def findrepo(p):
 
     return p
 
-def bailifchanged(repo, merge=True):
+def bailifchanged(repo, merge=True, hint=None):
+    """ enforce the precondition that working directory must be clean.
+
+    'merge' can be set to false if a pending uncommitted merge should be
+    ignored (such as when 'update --check' runs).
+
+    'hint' is the usual hint given to Abort exception.
+    """
+
     if merge and repo.dirstate.p2() != nullid:
-        raise error.Abort(_('outstanding uncommitted merge'))
+        raise error.Abort(_('outstanding uncommitted merge'), hint=hint)
     modified, added, removed, deleted = repo.status()[:4]
     if modified or added or removed or deleted:
-        raise error.Abort(_('uncommitted changes'))
+        raise error.Abort(_('uncommitted changes'), hint=hint)
     ctx = repo[None]
     for s in sorted(ctx.substate):
-        ctx.sub(s).bailifchanged()
+        ctx.sub(s).bailifchanged(hint=hint)
 
 def logmessage(ui, opts):
     """ get the log message according to -m and -l option """
diff --git a/mercurial/subrepo.py b/mercurial/subrepo.py
--- a/mercurial/subrepo.py
+++ b/mercurial/subrepo.py
@@ -464,12 +464,12 @@ class abstractsubrepo(object):
             return _("uncommitted changes in subrepository '%s'"
                      ) % subrelpath(self)
 
-    def bailifchanged(self, ignoreupdate=False):
+    def bailifchanged(self, ignoreupdate=False, hint=None):
         """raise Abort if subrepository is ``dirty()``
         """
         dirtyreason = self.dirtyreason(ignoreupdate=ignoreupdate)
         if dirtyreason:
-            raise error.Abort(dirtyreason)
+            raise error.Abort(dirtyreason, hint=hint)
 
     def basestate(self):
         """current working directory base state, disregarding .hgsubstate
diff --git a/tests/test-rebase-pull.t b/tests/test-rebase-pull.t
--- a/tests/test-rebase-pull.t
+++ b/tests/test-rebase-pull.t
@@ -78,6 +78,7 @@ Abort pull early if working dir is not c
   $ echo L1-mod > L1
   $ hg pull --rebase
   abort: uncommitted changes
+  (cannot pull with rebase: please commit or shelve your changes first)
   [255]
   $ hg update --clean --quiet
 
@@ -95,6 +96,41 @@ Abort pull early if another operation (h
   [255]
   $ hg histedit --abort --quiet
 
+Abort pull early with pending uncommitted merge:
+
+  $ cd ..
+  $ hg clone --noupdate c d
+  $ cd d
+  $ hg tglog
+  o  1: 'C2'
+  |
+  o  0: 'C1'
+  
+  $ hg update --quiet 0
+  $ echo M1 > M1
+  $ hg commit --quiet -Am M1
+  $ hg update --quiet 1
+  $ hg merge 2
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg pull --rebase
+  abort: outstanding uncommitted merge
+  (cannot pull with rebase: please commit or shelve your changes first)
+  [255]
+  $ hg update --clean --quiet
+
+Abort pull early with unclean subrepo:
+  $ echo s = s > .hgsub
+  $ hg add .hgsub
+  $ hg init s
+  $ hg commit -m "generated a subrepo"
+  $ echo a > s/a
+  $ hg -R s add s/a
+  $ hg pull --rebase
+  abort: uncommitted changes in subrepository 's'
+  (cannot pull with rebase: please commit or shelve your changes first)
+  [255]
+
 Invoke pull --rebase and nothing to rebase:
 
   $ cd ../c