Why does ReportViewer control or the Microsoft Report Designer think it needs a reference to my file / assembly?

I'm using VS 2010 to build a web app in ASP .NET 3.5 [C#]. I drop a ReportViewer control onto my web form. I go to the design view, I click on the smart tab on the ReporViewer control and choose "Design a new report". When i do this I get an error saying that the Microsoft Report Designer could not load the file or the assembly of a custom project that i've included in my application. I don't understand why this is happening. Why does the ReportViewer control or the Microsoft Report Designer think that it needs to find this file or assembly? They seem unrelated to me??