general relativity - Why can't a spin 0 graviton couple to a photon?

I read that a spin 0 graviton can't couple to traceless energy, such as a photon, in the introduction in the book The Feynman Lectures On Gravitation. Why is this true? What does it mean for a particle's energy to be "traceless"?

This has been my reference all along.

A spin 0 graviton can only couple to the trace of energy, and not to traceless energy.