Invalid setup on a non-virtual overridable in vb member: mock => mock[It.IsAny<string>()]

Вот такой код:

public interface IGate
{
    string Name { get; }
}
public interface ICommutator
{
    Dictionary<string, IGate> Gates { get; }
}
[TestMethod]
public void TestGates()
{
    ICommutator mock = new Mock<ICommutator>();
    Mock<ICommutator> instance = mock.Object;
    mock.Setup(c => c.Gates[It.IsAny<string>()].Name).Returns("K1");
}

При отладке теста возникает ошибка "Invalid setup on a non-virtual overridable in vb member: mock => mock[It.IsAny()]" в последней строке тестового метода.