--- a/mercurial/manifest.py	Thu May 05 11:33:02 2011 -0400
+++ b/mercurial/manifest.py	Thu May 05 18:05:24 2011 +0200
@@ -41,7 +41,8 @@
     def readfast(self, node):
         '''use the faster of readdelta or read'''
         r = self.rev(node)
-        if self.deltaparent(r) in self.parentrevs(r):
+        deltaparent = self.deltaparent(r)
+        if deltaparent != revlog.nullrev and deltaparent in self.parentrevs(r):
             return self.readdelta(node)
         return self.read(node)