The international guidelines of Solvency II prescribe a regulation which should help insurance industry mitigating undesired outcomes arising from the exposure to the systemic risks. In particular, the rules on Solvency Capital Requirements recommend to separately compute them for each risk factor, where for the longevity risk sub-module the Solvency Capital Requirement results by the change in net asset value (NAV) due to a longevity shock which actually assumes a permanent reduction of the mortality rates for all ages by 20%. Nevertheless, the data based on statistics coming from various national longevity indices differ from those deriving from the regulatory assessment of solvency, determining significant underestimations or overestimations: A basis risk comes from a questionable adequacy of the longevity shock. This paper contributes to the discussion on Solvency Capital Requirements by focusing on the main features of the potential basis risk which determines the inappropriate capitalization of insurance companies. Furthermore we analyze the sensitivities of the basis risk to different ages for better assessing the actual risk of insurance portfolios.