I am reading Zee's book "QFT in a nutshell". I have a question on the photon propagator computation. For a massive photon, consider the Lagrangian $L = -\frac{1}{4} F_{\mu \nu} F^{\mu \nu} + \frac{1}{2}m^2A_\mu A^\mu + A_\mu J^\mu$, then the path integral is $Z = \int dx ~L = \int dx ~\{ \frac{1}{2}A_\mu[(\partial^2 + m^2)g^{\mu \nu} - \partial^\mu \partial^\nu]A_\nu + A_\mu J^\mu \}$. From this we get the photon propagator $D_{\mu \nu} $satisfies $[(\partial^2 + m^2)g^{\mu \nu} -\partial^\mu \partial^\nu ]D_{\mu \nu}(x) = \delta^\mu_\nu \delta^{(4)}(x)$, solve this, $$D_{\nu \lambda}(k) = \frac{-g_{\nu \lambda} + k_\nu k_\lambda/m^2}{k^2 - m^2}$$, but I can not see why the numerator has a term $ k_\nu k_\lambda/m^2$?