 Emperor Gaozu's distant nephew Li Yuan (李瑗, different character than Emperor Gaozu's name) the Prince of Lujiang, who was then commandant at You Prefecture (幽州, roughly modern Beijing), was an ally of Li Jiancheng's, and when Li Shimin sent Cui to You Prefecture to summon Li Yuan to the capital Chang'an.Li Yuan became fearful, and at the instigation of his deputy Wang Junkuo (王君廓) started a rebellion.He seized Cui and tried to force Cui to reveal military secrets of the imperial army, but Cui refused.