With a lattice of isotropic point-oscillators as an idealized model, the error of the usual additive approximation for the van der Waals interaction in solids is estimated. For a few common crystals, the estimated errors for van der Waals energy and its first and second derivatives have values up to 13%, 34% and 70% respectively. The model moreover points to the possible importance of the long range dipolar interaction as a mechanism determining the structure of the exciton-band in. insulators.