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.

May be expression don't know which dataset to look at runtime you can spacify for ReportItems!ThbForex.Value and check if this not fix the issue the use another dataset with its repective fields. one more thing what i noticed is your using directly ReportItems!ThbForex.Value with Fields or First etc.