The electrostatic energy per degree of freedom of a two-temperature fully ionized plasma was calculated by Rostoker, using resonant integral approximations. His results include the contributions from plasma waves and ion waves. In this note it is shown that the non-resonant contribution from electrons, i.e. the contribution from other than plasma waves, is important for the same wavelengths for which ion waves are important. Also, the validity of the resonant approximations in the ion wave region is examined by comparing Rostoker's results with those obtained by computer calculation. Although Rostoker's results give good qualitative agreement, significant quantitative disagreement is found. A more accurate approximation procedure is presented which gives better agreement with computer results.