# Is the photon propagator always coupled to a conserved current?

I was reading Mandl and Shaw's book on quantum field theory and on page 184 I came across the sentence "the photon propagator always occurs coupled to conserved currents". This allows him to disregard the term proportional to $$k^{\mu} k^{\nu}$$ in the photon self energy $$\Pi^{\mu \nu} (k)$$. I understand this is true if the photon propagator in question is coupled with the external legs (which in the S matrix expansion would indeed be given by $$j^{\mu} (x) = N[\overline{\psi}\gamma^{\mu} \psi]_x$$). But in case it was coupled to internal lines how could it be coupled to conserved currents? It would just be coupled to two propagators like $$S_F(p_1)$$ and $$S_F(p_2)$$, with off-shell 4-momenta.