Multi DataSet - How to Count Specific Fields in Report body

Dear Sir,

I have two data set, some data set fields are counted in report body i did tried but i got Error below Expression i did wrote 

= Count(iif((Fields!TransitionID.Value=6),1,nothing))

i got Error 

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In case i have Single dataset this expression worked  = Count(iif((Fields!TransitionID.Value=6),1,nothing)) 

but have multiple dataset not worked 

Please help me 

Thanks 

Emalai