Unable To Cast A Partial Class Based On LINQ To SQL Object

I have a database table called Animals which I drag on to my Linq to SQL DBML file.

This creates me a class called Animal.

I then create a class to extend it, like so:

public partial class Animal{
//my extra code here
}

But what I also want to do is have a class like this:

public class Zebra : Animal{
//even more extra code here
}

The trouble is I get an InvalidCastException from my repository, when I do this:

Animal a = dataContext.Animals.Where(c=>c.id.Equals(id)).SingleOrDefault();
return (Zebra)a;

I've stripped back my classes to the point where they're completely empty in an attempt to work why it doesn't work. To no avail.

This casting process did used to work in the non-LINQ-2-SQL project that I'm switching to MVC/LINQ.