diff -r 935508184d47 -r b4a62d6f0353 hgext/evolve.py
--- a/hgext/evolve.py	Mon Jun 01 10:58:50 2015 -0700
+++ b/hgext/evolve.py	Mon Jun 01 12:29:12 2015 -0700
@@ -1398,9 +1398,12 @@
                    progresscb=None):
     """Stabilize a unstable changeset"""
     obs = orig.parents()[0]
-    if not obs.obsolete():
+    if not obs.obsolete() and len(orig.parents()) == 2:
         obs = orig.parents()[1] # second parent is obsolete ?
-    assert obs.obsolete()
+
+    if not obs.obsolete():
+        ui.warn("cannot solve instability of %s, skipping\n" % orig)
+        return False
     newer = obsolete.successorssets(repo, obs.node())
     # search of a parent which is not killed
     while not newer or newer == [()]: