As in summary. This is a regression from r95929 / bug 68783.
Created attachment 108679 [details]
This is an untested patch that I believe will address the issue. I’m putting it up as it may be a few hours before I’m able to test the patch, and it may be useful for anyone being troubled by the assertion.
Created attachment 108683 [details]
Dan says that this addressed the issue for him, so I’m putting it up for review.
Fixed in r95961.