I am running into an issue with the 'lookup' function in a textbox. My report has multiple datasets and I am assuming the issue is SSRS doesn't know what dataset to tie the first part of the expression to, but I cannot figure out how to fix it or if it's even possible.

This is the lookup statement, which works fine if I put it into a table that is tied to my first dataset ("Building"). It doesn't work when I place the same statement in a text box though. It gives me the error "Unknown collection member."

=Lookup(Fields!Construction_Code.Value,Fields!Construction_Code.Value,Fields!Value.Value, "Construction_type")

Is there a way I can signify the first Construction_Code value comes from the "Building" dataset?