Wendy, you're never being too picky! Thank you!

France/Mark - Can I suggest, either: Are you kidding me? or Is that a joke?

'Are you kidding?' and 'Are you joking' have very slightly different connotations. As when someone says something slightly unbelievable, and the other says, "are you kidding!' - these are more expressions of astonished and incredulous admiration.

Whereas: Are you kidding me? or Is that a joke? can be interpreted as a more serious question as to the other's intentions.
Of course the meaning can also change according to the intonation used in spoken language, but of course we can't hear that in the written language, so we need to use words in certain ways to convey this.

Sorry, I hope you don't think I'm being too picky!

Thanks, Mark.

And even then, I was only auditing classes…”
Stéphane – “Are you joking/kidding?