亡羊补牢 (亡羊補牢) wáng yáng bǔ láo

wáng yáng bǔ láo set phrase to fix the pen after the sheep have escaped; too late to solve the problem
Domain: Idiom 成语
Notes: Here 牢 means 'pen' (CC-CEDICT '亡羊補牢'; CCI '亡羊补牢', pp. 65-66; Guoyu '亡羊補牢')