Fatal error after running cron and re-indexing

Issue Summary

After manually running cron.php and re-indexing my site's content I get a fatal error when I try and view unpublished nodes. The message is:
Cannot use assign-op operators with overloaded objects nor string offsets in .../includes/menu.inc on line 356

Manually re-publishing the nodes still brings up the error.
On the other hand, if I manually unpublish a published node and then try to view it, I don't get the error.

I'm wondering whether it has something to do with node expire because it was the first time I ran cron and so the first time my expired nodes were unpublished. Haven't had this error before then.