Niu Sengru
On a day in spring 831, when Li Zaiyi was feasting with an imperial messenger, Yang Zhicheng and a group of soldiers he gathered started a disturbance. Li Zaiyi and his son fled to Yi Prefecture (易州, in modern Baoding, Hebei), in neighboring Yiwu Circuit (). As part of his takeover, Yang also killed the prefect of Mo Prefecture (莫州, in modern Cangzhou, Hebei), Zhang Qingchu (). Then-reigning Emperor Wenzong was initially concerned and considered his options, but at the urging of the chancellor Niu Sengru, who pointed out that for decades, Lulong had, in effect, not been under the control of the imperial government, and that Li Zaiyi himself had taken control of the circuit without imperial input. Emperor Wenzong thus allowed Yang to remain in control of the circuit and made him acting military governor (留後, "Liuhou"). Later in the year, Emperor Wenzong made Yang military governor.