What does error "crash" mean?

I’m seeing crash entries in my history.txt, for example:

3	:ok	:poll	[[:poll {"429" [[1 2]]}]]
2	:invoke	:crash	nil
13	:invoke	:crash	nil
3	:invoke	:crash	nil

Also in my results.edn:

:crash {:valid? false,
        :count 10,
        :ok-count 0,
        :fail-count 0,
        :info-count 10},

These are also contributing to high latency.

I understand it’s used as a catch-all for internal errors but I’m at a loss for what might be causing it, as there’s nothing in the nodes’ logs that tell me what the issue is (there’s no error with code 13).

I’m working on challenge #5c, if that counts for something.