Wenbo Mao has kindly pointed out to us that the collusion attack in section 3.2 does not work in the case that the parameter a lies in Z_n^*. (This is because all such elements, by definition, cannot have order any multiple of w.) Therefore, to make this attack work, the group manager should choose a to live only in Z_pq^* where, in the attack, n=pqw. Notice that without the factorisation of n it cannot be verified what is the exact order of a.