  In the Standard Model extended by sterile neutrinos, modified W-l-nu
couplings arise, which are able to induce a tree-level enhancement to lepton
flavour universality violation in kaon decays. The additional mixing between
the active neutrinos and the sterile ones can generate deviations from
unitarity in the leptonic mixing matrix for charged currents. We reconsidered
this idea in the context of the inverse seesaw and evaluated its impact on the
well measured ratio $R_K$. We show that the current experimental bound can be
saturated in agreement with the different experimental and observational
constraints. Similar results can be obtained when considering the ratio
$R_\pi$.
