我用khb做中介效应分析，然后想用bootstrap样本自抽样，但是命令报错，具体如下

bootstrap _b[Diff], saving(med, replace) reps(500) seed(9876) : khb mlogit health (socia self) (Comscores)
(running khb on estimation sample)
option baseoutcome() is required with prefix bootstrap;
an error occurred when bootstrap executed khb
r(198);