diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -343,6 +343,8 @@ class revlogoldio(object):
         return index, nodemap, None
 
     def packentry(self, entry, node, version, rev):
+        if gettype(entry[0]):
+            raise RevlogError(_("index entry flags need RevlogNG"))
         e2 = (getoffset(entry[0]), entry[1], entry[3], entry[4],
               node(entry[5]), node(entry[6]), entry[7])
         return _pack(indexformatv0, *e2)