APEX bone reference error with a mesh that has no bones

I’m trying to simulate an entire object as cloth, and I created a .apx file for it, but when I import it, I get:
This file references bones which do not exist in the Skeletal Mesh:

RootNode

Proceed?

and the cloth will not simulate. I don’t understand. There’s no armature, no bones, and there weren’t any when I created the .apx file. What gives? How do I get this to work?

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