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In the spirit of sharing solutions to the same problem...
I had the same problem but it wasn't fixed by the [Key] solution when coding a MVC 4 app with VS2012.
I forgot to include the getter and setters on my model members. Just having them public isn't enough and Visual Studio will give you the same error message.
I have identified 5 scenarios that result in this message - covered in a small blog post about it.