Why Is the Photon One-Point Function Zero in QED?

[taishizhiqiu]

In 'an introduction to quantum field theory' by peskin, he writes: To analyze the photon one-point function, note that the external photon must be attached to a QED vertex. Neglecting the external photon propagator, this amplitude is therefore:

I really cannot justify this equation. Can anyone explain it to me?

 

[Conformal Bear]

Intuitively, the expression is essentially the diagram with the two fermion lines and vertex (by definition of $j_{\mu}$). Note the, "neglecting the external photon propagator." If you include the photon propagator as a factor, the expression gives rise to the same diagram as if the current were to be replaced by the photon field, i.e. the photon one-point correlation function.