Filtering on NULL in CR

I really hate to ask such a simple question here but I have searched the forum, bought books, and tried to get help from SAP Support without any luck. I've tried all of what seem to be the obvious choices such as search on NULL, empty string ("" or '' ) and nothing is working. I've also tried using string functions as opposed to just using the Select Expert with the same results. If anyone can point me in the right direction it would be much appreciated. And if there is a more appropriate forum or online resource I would love to know about it (I thought buying "Crystal Reports 2008, The Complete Reference" would have sufficed, but I can't find anything in there on testing for an empty field, which I find incredible).