Error in MVC 4 Application that uses OAuth in Visual Studio 2012

I'm writing an MVC 4 Application that uses OAuth in Visual Studio 2012. I can compile without errors, but every time I run the application (previously working) I get the following error message.

Could not load file or assembly 'System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

I haven't directly changed that package (System.Web.WebPages.Razor).