I have created several reports without an issue. Though if I use interactive sorting with a report that has 2 fields in the document map properties under edit details group like the following :
=Fields!FirstName.Value &" " & Fields!LastName.Value

everthing populates as it should when I open up the report. If I click the sort button on the report I recieve the following error : An error occured during local report processing. An error occured during  report processing. An Internal error occurred on the report server. See the error log for more details. I would like to sort the fields in the report.

If I have 1 field in the document map properties there are no issues. Is there a way to fix this issue