✅ Covariant return type properties

Am I missing something - I thought that property return types could be covariant if they were readonly. So why does this not compile?

public abstract class C
{
    public abstract object Prop { get; }
}

public class D : C
{
    public override int Prop { get; }
}

public abstract class C
{
    public abstract object Prop { get; }
}

public class D : C
{
    public override int Prop { get; }
}