Custom SSRS Report errors out

I built a new SSRS report based off a (working) preexisting one. I test ran the new one in Preview mode of Visual Studio and got the correct data returned. So, I implemented it and set everything up in all the usual places. But when I attempt to run it in Tessitura, the report errors out with this error:

"There was an error processing the request.

Exception of type 'System.Web.HttpUnhandledException' was thrown. The given key was not present in the dictionary."

Then a bunch of "at System.Web..." lines.

Checking Report Manager, the report I'm working with is there (good), and I can even run it and get data. 

So, I have a working report that will work in two different places, just not the useful place. Does anyone have any idea what the error causer could be?


Thanks!