How do I cast a derived class to its generic parent type?

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C#

This question is entirely in the code below. I have a derived class that I cannot cast to its generic parent type and I don't understand why.

public interface ISession {}

public abstract class SessionBase : ISession, IEnumerable<ISessionState<ISession>>
{
    public List<ISessionState<ISession>> SessionStates = new List<ISessionState<ISession>>();

    public IEnumerator<ISessionState<ISession>> GetEnumerator() => SessionStates.GetEnumerator();

    IEnumerator IEnumerable.GetEnumerator() => SessionStates.GetEnumerator();

    public void RunStateMethod()
    {
        Console.WriteLine( SessionStates[ 0 ].SomeMethod() );
    }
}

public class SessionA : SessionBase
{
    public SessionA()
    {
        var t1 = new SessionAState( this );
        var t2 = t1 as ISessionState<ISession>; // t2 is null
        SessionStates.Add( t2 );
    }
}

public interface ISessionState<S> where S : ISession
{
    string SomeMethod();
}

public abstract class SessionStateBase<S> : ISessionState<S> where S : ISession
{
    public SessionStateBase( S session ) { Session = session; }

    public ISession Session { get; } = null;

    public abstract string SomeMethod();
}

public class SessionAState : SessionStateBase<SessionA>
{
    public SessionAState( SessionA session ) : base( session ) {}

    public override string SomeMethod() => $"Some method in {nameof(SessionAState)}";
}

What I have tried:

I have tried reading about covariance and contravariance. I have also tried experimenting with various casts and use of the in and out generic keywords.

I have come to realise that my brain just doesn't seem to be efficiently wired for problems of this type, so please feel free to point out any complete dumbness on my part. Any help or advice will be very gratefully received.