Update: why is dnk an illegal name for kerning groups?!
Lines 15 and 39 from features.fea in the Temp folder:
15 @MMK_L_dnk = [ordfeminine mu zero.tf one.tf …
39 @MMK_R_dnk = [ordfeminine mu zero.tf one.tf …
EDIT: Solved! The dnk name was not the problem. The problem was the NULL glyph is in this group. I found this thread where Tim mentioned a similar problem. My script was using dnk for everything that doesn’t need to be kerned, including things like zero-width and control characters.