Blank values causing set analysis issue

[#1]

I have the following expression

=COUNT(DISTINCT {$<Firm-={"ICAP","*TEST*"}>}SessionId)

Unfortuantely records where the Firm field is blank are not being counted. How can i get them counted, whilst simutaneously ignoring the two values stated above?

[#2]

A variation of this worked for me, but I can't help feel the expression could be slightly more straightforward

[Nicole-Smith]

Single quotes are used for strings, double quotes are used for field names.