--- a/mercurial/cmdutil.py	Sun Dec 27 23:37:14 2015 +0900
+++ b/mercurial/cmdutil.py	Mon Jan 04 16:14:04 2016 -0800
@@ -1308,7 +1308,7 @@
             diff = self.diffopts.get('patch')
             diffopts = patch.diffallopts(self.ui, self.diffopts)
             node = ctx.node()
-            prev = ctx.p1()
+            prev = ctx.p1().node()
             if stat:
                 diffordiffstat(self.ui, self.repo, diffopts, prev, node,
                                match=matchfn, stat=True)