  If baryons couple only with $\omega $-mesons, one found the baryon spectral
function may be negative. We show this unacceptable result is caused by the
$k_\mu k_\nu $-terms in the $\omega $-meson propagator. Their contribution may
not vanish in approximate calculations which violate the baryon current
conserves. A rule is suggested, by which the calculated baryon spectral
function is well behaved.
