diff --git a/mercurial/dirstate.py b/mercurial/dirstate.py
--- a/mercurial/dirstate.py
+++ b/mercurial/dirstate.py
@@ -256,6 +256,10 @@ class dirstate(object):
 
         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