In an incomplete regulation framework, the regulator cannot replicate all the possible outcomes by himself since he has no influence over some firms in the market. Due to asymmetric information, it may be better for the regulator to allow the unregulated firms to extract a truthful report from the regulated firm through side-payments under collusion, and therefore the "collusionproofness principle" may not hold. In fact, by introducing an exogenous number of unregulated firms, social welfare differences seem to favour a collusion-allowing equilibrium. However, such result will depend on the relative importance given by the regulator to the consumer surplus in the social welfare function.