在用stata进行opreg时，命令如下：
sort code obs
by code: gen exit=1 if obs[_n+1]-obs!=1
egen mis = rowmiss(exit)
replace exit=0 if mis
drop mis
replace exit=0 if obs==2017
label define exit 1 "exit" 0 "survive"

opreg ln(y),exit(exit)state(lnk lnl) proxy(misk) free(lne) cvars(obs) vce(bootstrap, seed(1357) reps(5))


结果总是出现如下错误：

factor-variable and time-series operators not allowed
an error occurred when bootstrap executed opreg

求助各位学长