It turns out it was not a combined filed but that I had at some time put those ztd into dupdate as well as dlupdate. Rather than open the dlupdate to add those ztd I tried to add them "a step above" and missed the correct folder the first time. I'm lucky it was that folder not one farther down the list.
Thanks for the suggestions Heather, Zawarotto, Heather, and Jay.