SqlException (0x80131904):无效的对象名“dbo.Categories”

I am getting the exception above when I run an application. The application is using asp.net mvc 3 / C#. I made an mdf file and added it under App_Data folder in Visual Web Developer Express. I added connection strings to the web.config folder but when I run and browse to /store, I get the error above with the line I am getting the exception above when I run an