linux-dmabuf: Is it an error to specify different modifiers for different planes?

Weston, at least, refuses to attempt to import a buffer where different planes have different modifiers set. This seems reasonable, as to the best of my understanding the format+modifier pair is a property of the whole logical buffer, which implies that each plane must have the same modifier (and same format, as that's only set once).

Should zwp_linux_buffer_params_v1.add send a protocol error if the supplied modifier is not the same as previous modifiers? Should add lose its modifier parameters, in favour of sending them with the create/create_immed request, as format is?