I know this error/crash is introduced because of lame coding practice but nevertheless it results in a segfault and a coredump.
If an expire_function is missing a return time interval bro would segfault.
The attached script demonstrates this.
the issue becomes very subtle and difficult to identify/debug if one has complicated expire_func where different conditions return different times for extending or deleting values in the table. The segfault may not happen for days then. Lack of backtrace and any other indicator results in identifying and debugging this issue very hard.
FreeBSD 10.3-STABLE #0 r314188