This (The parameters dictionary contains a null entry for parameter) exception was thrown at me and confused me for a while.
If you ever encounter it I can almost guarantee that you have a Routing-issue. That is the way the ASP.NET MVC framework is figuring out which controller and which method to invoke.
In my case I had a controller action method that required two parameters but my routing table only allowed for one.
Here is a description on how to add your own, custom route rules.
And also here is a great tool to understand and debug your routing-table entries.
Finally – this webcast contains a discussion about how to change routes.