Lookup issue in SSRS

HI All,

I have small issue in using Lookup function. I wanna use ReportItems!textbox4 this in lookup function

I have seen some where that If we want to use this ReportItems function then both datasets should be group by same Coloumns.

But in my case dataset1 grouped by tasknumber and  dataset2 grouped by NPRNumber Coloumn.

So I wanna pull one calculated values from dataset2 to dataset 1. so in this case I should have to use ReportItems function 

but LOOKUP is not able to accept this.

=Lookup(Fields!NK_UPRN.Value,Fields!NK_UPRN.Value,ReportItems!Textbox4,"Test")

If there is any other way for this please let me know

Thanks in advance