Mocking a method that has optional parameter

I have a method public void SendMail(string email, string subject, string body, List<Attachment> attachements = null) where last parameter can be null or List<Attachment>.

I mocked it like this:

        var mailMock = new Mock<IMailService>();
        mailMock.Setup(m => m.SendMail(It.IsNotNull<string>(), It.IsNotNull<string>(), It.IsNotNull<string>(), It.IsAny<List<Attachment>>())).Verifiable();
        mailMock.Verify();

But it doesn't allow null values in attachment parameter. How can I allow both null and List<Attachment> to be passed?