User281315223 posted
You'll need to make sure you have the proper references as well (especially if you have upgraded your ReportViewer recently).
You may want to check out this ASP.Net thread, which discusses a similar issue and a
Stack Overflow discussion that may help as well.
You may need to make sure that you have the 10.0 Version of ReportViewer as well if you haven't downloaded or installed it. The download can be found
here.