From a study of the lineshape of the X(3872), the LHCb collaboration measures a sizeable negative effective range. This cannot be reconciled with a shallow D D * bound state hypothesis. Based on Weinberg's compositeness criterion, together with a theorem by Smorodinsky, it follows that the X has to have a compact hidden charm structure, most likely a tetraquark, interacting with unbound D D * pairs via short-distance color forces. This conclusion is strengthened by the general pattern recently emerging for exotic mesons.