  Alvarado, Aranda, and Bonilla propose a sub-GeV U(1)R gauge boson model to
explain the proton charge radius discrepancy. Their model assumes opposite sign
U(1)R charge assignments for up and down quarks in the nucleon. I point out
that might cause a difference between neutron and proton mass radii that
contradicts existing p-p and n-p scattering data. Using existing neutron and
proton scattering data I show that the neutron and proton mass radii are equal
within about 2%. This constitutes a challenge to models that attempt to explain
the discrepancy by modifying the quark couplings.
