public void TestGetAgeDifferenceError()
        {
            var personList = new List <Person> {
                new Person {
                    Name = "George", Gender = "Male", DateOfBirth = Convert.ToDateTime("01/01/2001")
                }, new Person {
                    Name = "Beck", Gender = "Male", DateOfBirth = Convert.ToDateTime("01/01/2000")
                }
            };

            _addressBookLoader.Setup(c => c.LoadAddressBook()).Returns(personList);

            classUnderTest = new AddressBookService(_addressBookLoader.Object, _logger.Object);
            var result = classUnderTest.GetAgeDifference("Beck", "Jack");

            Assert.Null(result.Result);
        }