isearch and wrong-type-argument hash-table-p nil ?

[Eric Abrahamsen <eric@ericabrahamsen.net>]

Off and on for the past week or so (I'm using Emacs from git and, erm, a
lot of packages), I've been getting this error message in the minibuffer
any time I type in a search string that isn't found:

I-search: <my not-found string> [(wrong-type-argument hash-table-p nil)]

After which i-search is mostly broken: I get that error no matter what I
search for, and nothing is found. None of the usual debug toggles will
trigger.

I'll start the bisect process as soon as I've been sufficiently annoyed,
but I did want to pop this up here and see if it looks familiar to
anyone.

If it doesn't, I'll reply to my own message with the fix to the problem,
once I've found it.

Thanks,
Eric