Add a Not Null expression in MDX

Hello,

I have a KPI with this expression:

IIF(([Measures].[Fact Service Request Count],[Fact Service Request].[Be In KPIS].&[1],[Fact Service Request].[Applicant].&[1],[Fact Service Request].[Valid Humanity].&[1], [Subscriber].[KInd].&[1],[Fact Service Request].[Be Control].&[1],[Fact Service Request].[Control Result].&[1])=Null,0,([Measures].[Fact Service Request Count],[Fact Service Request].[Be In KPIS].&[1],[Fact Service Request].[Applicant].&[1],[Fact Service Request].[Valid Humanity].&[1], [Subscriber].[KInd].&[1],[Fact Service Request].[Be Control].&[1],[Fact Service Request].[Control Result].&[1]))

I wanna add this expression that "[Fact Service Request].[Payment Date] is not null".

How could I write it?

Thanks in advanance.