Aggregate in SSRS

Question

I created an expression to sum all rows, please see below:

=SUM(SWITCH(Fields!LAK.Value > 0, Fields!LAK.Value/ReportItems!ThbForex.Value, Fields!LAK.Value = 0, Fields!THB.Value))

ReportItems!ThbForex.Value comes from a different dataset. But, when I run the report, it gives me the error below:

 

Can someone help me with this? Thanks.