malformed regular expression

I'm getting this run-time error:

<some time> rune-time error: attempt to match to a malformed regular

The problem is that I don't know where it occurs. (in which event, ...)
Do I get this message when trying to write to a file a empty string returned
by Bro?
Is there a way to get more precise error message? To be able to correct my

As usual, when reporting a problem like this, please include your script(s)
so it can be reproduced.