  Zhang and Kobayashi (2004) attempted to calculate early afterglow emission
from a system of forward and reverse shocks in GRB outflows for the case of
magnetized ejecta. We point out a fundamental error in the underlying dynamical
model. According to the authors, energy and momentum carried by the magnetic
field of the ejecta are not transfered to the forward shock. This is an
incorrect assumption that invalidates the results.
