--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -411,6 +411,8 @@ class revlog:
# retrieve the parent revision of the delta chain
chain = data[24:44]
+ if not chain in self.nodemap:
+ raise "unknown base %s" % short(chain[:4])
# track the base of the current delta log
r = self.count()