diff -r 2e47e461d2ca -r d259322a394b mercurial/dirstate.py
--- a/mercurial/dirstate.py	Fri Sep 05 15:36:56 2014 -0700
+++ b/mercurial/dirstate.py	Fri Sep 05 11:37:44 2014 -0700
@@ -256,6 +256,10 @@
 
         See localrepo.setparents()
         """
+        if self._parentwriters == 0:
+            raise ValueError("cannot set dirstate parent without " +
+                " calling dirstate.begingparentchange")
+
         self._dirty = self._dirtypl = True
         oldp2 = self._pl[1]
         self._pl = p1, p2