I had completely janked up my if logic. All sorted now!

Check your previous returns and try to see why it's returning early, or why it's not reaching the 'else' part. Console.WriteLine is your friend.

I think you're exactly right, tried changing it to return a literal string and it's still labeled String.Empty, thank you for your help :)

Your code must be not reaching that line, because null and empty string are 2 different things.

Sorry if this is just me being dumb, I'm stuck with the error:
Test Failed
Expected: null
But was: <string.Empty>

I am returning a null value with : return null;

Is this not the way to do it in C#? I've tried googling but found nothing but discussion on the difference between null and String.Empty, which doesn't exactly help me.