--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -143,7 +143,7 @@ class lazyparser(object):
                 # the revlog may have grown since we've started running,
                 # but we don't have space in self.index for more entries.
                 # limit blocksize so that we don't get too much data.
-                blocksize = self.datasize - blockstart
+                blocksize = max(self.datasize - blockstart, 0)
             data = self.dataf.read(blocksize)
         lend = len(data) / self.s
         i = blockstart / self.s