SSRS and the Fountain of Useless Errors: Conversion from type 'CalculatedFieldWrapperImpl' to type 'Integer' is not valid

Hey, I'm no novice. I've been doing this computerish stuff for a good long while. But Microsoft SQL Server Reporting Services (SSRS) stumps me on a regular basis. I use BIDS (Business Intelligence Design Studio) and SSDT (SQL Server Data Tools) on a regular basis -- weekly, sometimes daily. It seems like it gives the worst, most cryptic, most useless error messages of all time. Well, besides the error messages that come from Linux, of course. 

Here's the doozie I got today:

The Sort expression for the grouping 'FUBAR' contains an error: Conversion from type 'CalculatedFieldWrapperImpl' to type 'Integer' is not valid.

So what had happened was this ... I had built a query (a view actually) in SQL Server and added it to the report and used it in a matrix. Then I decided I needed more columns, so I modified the view and then modified the data set. Then I tried to use the new columns to sort a group. When I tried to preview, I got this gloriously useless message.

I tried a lot. Googling, Binging, whatever else one does in these situations. I took a coffee break and thought about it. I then realized that perhaps since I changed the source of the data that the data cache had gone bad. So I found the file report.rdl.data in my project folder and deleted it. 

That did the trick.

Remember kids, anytime you change the underlying data source, delete the cache from BIDS or SSDT. It will cure a huge number of problems in SSRS.