Sorry, yams are NOT "real" sweet taters. If you like them, fine. I have to have the real deal. Nice recipe. They are hard to come by around here. If yams trip your trigger, so be it. I made a few pie's with yams one time. (FAIL) Think it may be a sugar thing in the tater. Just a thought.
Actually, what are marketed as "yams" in the average grocery store in the U.S. are in fact sweet potatoes. True yams have more starch and are much lower in sugar. Perhaps you got hold of some real yams.
I used "garnet" which, again, is indeed a sweet potato.