_penalty gist method invoked with one key NULL

As in subject.
What it does, it gets through picksplit, I return good values, valid unions,
etc. Than (I guess) postgres is trying to insert another value in tree, hence
penalty is called. Why one of the values penalty is called with is NULL, and
I have no idea if that's valid.
From all the examples available in contrib dir, I can deduct that it shouldn't
happend.
Now question, how shall I debug postgres in this case?
can I just run postgres, in non forking mode, on gdb?
I am running psql with all commands required to create new type, functions,
operators, and gist. So there must be separate connection available for that.

Thanks.

--
GJ

Binary system, you're either 1 or 0...
dead or alive ;)