gistfile1.cs

namespace Lib
{
    public interface Counter { int Count(string s); }

    public abstract class AbstractClass : AbstractParent
    {        
        public abstract int Foo(int x);
        public virtual int Foo(string s) { return Foo(Counter.Count(s)); }
    }

    public abstract class AbstractParent
    {
        internal abstract Counter Counter { get; }
    }
}

[Test]
public void Test()
{
    // this line threw the exception in the first email
    var instance = Substitute.For<Lib.AbstractClass>();
}