Congratulations to the 200th post for this kata. :-)

If you changed that if, you probably got the wrong answer on the last if instead. Neither remString nor remString2 will change from their initial values.

(Btw. That last if-statement will result in a a false negative for isMerge("aaaa", "aa", "aa") as well, so you might need to rethink your strategy.)

Well, an empty string can absolutely be the merge of two empty strings. Just like 0 + 0 = 0. I haven't gone through the whole code, but the first if-statement looks suspicious.

Good luck! :-)