！！！求助大家，请问一下我运行下面一串指令想做did模型回归，为什么会出现Invalid name呢？

use"C:\Users\acer\Desktop\stata\nlswork.dta", clear

xtset idcode year

global xlist 'grade age ttl_exp tenure not_smsa south race'

gen time=(year>=77)&!missing(year)

gen treated=(idcode>2000)&!missing(idcode)

gen did=time*treated

reg ln_wage did time treated $xlist