Why does the Using statement give an error even though my type implements IDisposable?

In my DataAccess project I have the code:

using (TandAEntities dataContext = new TandAEntities())
{
    // Some code
}

This gives no error and works fine.

But in my Unit Test project, I have that same code, but the compiler gives an error; type used in a using statement must be implicitly convertible to 'System.IDisposable'

I checked the definition by pressing F12, and it does indeed implement IDisposable, and has a dispose method.

Am I perhaps missing some references?