Not a suggestion (As long as it solves the tasks within the time constraint, it is a valid solution)

Yes, absolutely true and it is preventing me from completing the Kata because of a null.

I thought it was a kata issue, thank you for the tip

C#:
test structure are as follows
Assert.AreEqual(expected, Kata.TitleCase(sampleTitle, sampleMinorWords));
where Kata.TitleCase should contain 2 arguments (the original title and list of words to check in one string)

Test cases in test 1 goes fine as expected/sampleTitle/sampleMinorWords are present
While test 2 and test 3 don't have any sampleTitle, only expected and words list

You have to put it like string.Join(''), as in some other methods you have to write System.X instead of X. Annoying and common on codewars.