"Operand data type nvarchar is invalid for minus operator" in SSRS query designer/refresh fields

I have a valid query 开发者_如何学Python(tested in SSMS and also running fine in report Preview) however it will not update the dataset with the fields.

How can I force it to do so without manually entering all the fields?

Currently, if I click "Refresh Fields" I get the above error returned

Although the report/dataset parameters were correctly defined, in order to get the fields returned, you have to hard code a bunch of

DECLARE @BILLGATES VARCHAR(100) = 'BEL-LEND'

statements at the top of the query, then remove them once you've refreshed the fields

(this is within the query windows in the dataset definition, also poor is the fact that once you click 'refresh fields' there is no visual indication of any means to indicate success/failure/anything)