I create JSON strings in my web services - but I'm not sure how MVC wants to have it delivered so I'll wait to see if no one else gives you assistance.

But I will share with you a really great web site that validates JSON strings

http://jsonlint.com/

There is a lot of "escaping" that goes on in JSON strings to make them web-transmittable - and that web site will take a JSON string that is giving you a problem and parse it and tell you exactly where you are missing a } or a ] character.