平行趋势检验变量回归报错

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各位大侠，做平行趋势检验作图时，运行
****************数据预处理****************
gen period = (year>=2010) & !missing(year) //设置虚拟变量，政策执行时间为2010年
gen treat=(city>15) & !missing(city) //生成地区为16的虚拟变量
****************2.coefplot命令画图****************
gen Dyear = year-2010
gen Before3 = (Dyear==-3&treat==1)
gen Before2 = (Dyear==-2&treat==1)
gen Before1 = (Dyear==-1&treat==1)
gen Current = (Dyear==0&treat==1)
gen After1 = (Dyear==1&treat==1)
gen After2 = (Dyear==2&treat==1)
gen After3 = (Dyear==3&treat==1)
gen After4 = (Dyear==4&treat==1)
gen After5 = (Dyear==5&treat==1)

**将以上交互项作为解释变量进行回归
先设定面板变量
xtset ln_iva year
xtreg ln_iva period treat Before3 Before2 Current After1 After2 After3 After4 After5 i.year, fe r

系统报错“the panel variable ln_iva may not be included as an independent variable”
各位大神问题出在哪里了？如何解决？

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关键词：CURRENT missing specify Before period