  Applying perturbation theory to the path-integral representation for the
mutual information of the nonlinear communication channel described by the
nonlinear Shr\"{o}dinger equation (NLSE) with the additive Gaussian noise we
analyze the analytical expression for the mutual information at large
signal-to-noise ratio ($\mathrm{SNR}$) and small nonlinearity. We classify all
possible corrections to the mutual information in nonlinearity parameter and
demonstrate that all singular in $\mathrm{SNR}$ terms vanish in the final
result. Furthermore our analytical result demonstrates that the corrections to
Shannon's contribution to the mutual information in the leading order in
$\mathrm{SNR}$ are of order of squared nonlinearity parameter. We outline the
way for the calculation of these corrections in the further investigations.
