public void CreateLetterHeadAsync_WhenLetterHeadIsNull_ThrowsArgumentNullException()
        {
            ICommonRepository sut = CreateSut();

            ArgumentNullException result = Assert.ThrowsAsync <ArgumentNullException>(async() => await sut.CreateLetterHeadAsync(null));

            // ReSharper disable PossibleNullReferenceException
            Assert.That(result.ParamName, Is.EqualTo("letterHead"));
            // ReSharper restore PossibleNullReferenceException
        }