Problem with Mandl & Shaw's Photon Propagator?

[Vic Sandler]

The problem is on pages 323 and 324 of the second edition.

Homework Statement

Given the lagrangian
$$\mathcal{L} = -\frac{1}{4}F_{\mu\nu}(x)F^{\mu\nu}(x) - \frac{1}{2}(n_{\mu}A^{\mu})^2$$
show that the momentum space photon propoagator is given by
$$D_F^{\mu\nu}(k) = \frac{-g^{\mu\nu} - k^{\mu}k^{\nu}(n^2 + k^2)/(kn)^2 + (n^{\mu}k^{\nu} + n^{\nu}k^{\mu})/(kn)}{k^2 + i\epsilon}$$

Homework Equations

The Attempt at a Solution

I can solve this problem if I replace the factor $(n^2 + k^2)$ with $(n^2 - k^2)$.

My question is this:

Should the book say $(n^2 - k^2)$ and not $(n^2 + k^2)$?

This question and this question only. The meat of the answer will be one word.