Light does not always travel on null geodesic?

I am currently reading Wald's General Relativity and a result of section 4.3 stomped me. Part of Maxwell's equation in GR may be written as $$\nabla^a F_{ab} = \nabla^a \nabla_a A_b - R^a_b A_a= 0.$$

Now, let us look for wave solutions, $$A_a = C_a e^{iS}$$, where we assume that the derivatives of the amplitude $$C_a$$ are "small" (geometric optics approximation), so that terms like $$\nabla^a\nabla_aC_b$$ may be neglected. We then obtain the condition that $$\nabla^aS \nabla_a S = 0.$$ In other words, the surfaces of constant phase $$S$$ are null and thus, by a well-known result, $$k_a = \nabla_a S$$ is tangent to a null geodesic. This just means that light travels on null geodesics. However, to get there, one had to assume that derivatives of $$C_a$$ were "small". In other words, the general case seems to suggest that in general, the surfaces of constant phase are not necessarily 0 and thus not null.

I, however, though, that light would always travel along null geodesics, without any kind of approximation?