I've created a ticket; I'll attach the report and classes to that ticket, but here's the step by step instructions on how I create the issue.
1) Create a Project Data Source
a) Create a new class object (.cs) file.
b) Inherit the Telerik.Reporting..SqlDataSource object
c) Rebuild the project and then right-click and view Designer.
d) Click Configure Datasource.
e) Choose Data Connection (choose existing data connection). We only have one connection string in the project
f) Use as Shared Connection
g) Select Statement or Stored Procedure and click Finished.
2) Create a Report using a stored procedure that requires Parameters.
a) Right-Click on the Report and choose Properties and click the ... next to "ReportParameters"
b) In Report ParameterCollection Editor Add a New Parameter.
c) Under AvailableValues->DataSource dropdown, choose the Project Data Source. This will create a new DataSource in (current datasource)
d) Try previewing the report. This is where I get the error.
(Note: if I recreate the parameter data source as a Current Data Source, rather than using the Project Data Source (following the wizard as above), it'll work fine.