Covariance with interfaces

Hi,

I have defined an interface as follows:

public interface IMyInterface<out T>
{
}

I have marked the generic parameter T as 'out' to indicate it should be covariant (that is, may also accept instances derived from T as well as T itself).

I think I misunderstand how far the compiler infers relationships, because on doing the following:

public static IMyInterface<T> Join(IMyInterface<T> first, IMyInterface<X> second) where X : T
{
    IMyInterface<T> joined = null;

    List<IMyInterface<T>> list = new List<IMyInterface<T>>();
    // THIS IS THE LINE THAT STATES INVALID PARAMETERS
    list.Add(second);

    // Some processing code
    
    return joined;
}

I was expecting the list.Add(second) to accept parameters of type IMyInterface<X>, because I have indicated that X derives from T and IMyInterface is covariant with respect to 'T' as indicated by the 'out' parameter.

However the code refuses to compile, saying that list.Add(second) has invalid parameters.

What have I assumed incorrectly from the behavior of covariance with respect to 'T' here?