NetworkTransformChild "root must have NetworkTransform" error

I've got a GameObject with a NetworkIdentity, a NetworkTransform, and a NetworkTransformChild. That GameObject has a child GameObject, and when I try to assign the child object to the 'target' property of the NetworkTransformChild component I get the error message "NetworkTransformChild root must have NetworkTransform".

I'm not sure what that error means. The documentation seems to suggest that how I've set it up is correct. Does anyone know what I'm doing wrong?

Sam