diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -592,10 +592,10 @@
         self._storedeltachains = True
 
         self._io = revlogio()
-        if rustrevlog is not None and self.opener.options.get(b'rust.index'):
-            self._io = rustrevlogio()
         if self.version == REVLOGV0:
             self._io = revlogoldio()
+        elif rustrevlog is not None and self.opener.options.get(b'rust.index'):
+            self._io = rustrevlogio()
         try:
             d = self._io.parseindex(indexdata, self._inline)
         except (ValueError, IndexError):