The situation where conversion to Islam was penalized in an Islamic state could not last, and the devout Umayyad caliph Umar II has been credited with changing the taxation system. Modern historians doubt this account, although details of the transition to the system of taxation elaborated by Abbasid-era jurists are still unclear. Umar II ordered governors to cease collection of taxes from Muslim converts, but his successors obstructed this policy. Some governors sought to stem the tide of conversions by introducing additional requirements such as undergoing circumcision and the ability to recite passages from the Quran. According to Hoyland, taxation-related grievances of non-Arab Muslims contributed to the opposition movements which resulted in the Abbasid revolution. In contrast, Dennett states that it is incorrect to postulate an economic interpretation of the Abbasid revolution. The notion of an Iranian population staggering under a burden of taxation and ready to revolt at the first opportunity, as imagined by Gerlof van Vloten, "will not bear the light of careful investigation", he continues.