在做空间杜宾检验时，出现以下的的结果，lk的标准差和z值等出现了空值，问题出在哪里？应该怎么解决？stata代码如下：
gen lgdp=log(gdp)gen ls=log(s)
gen ld=log(d)
gen ll=log(l)
gen lk=log(k)
gen lfdi=log(fdi)
gen lgov=log(gov)
gen lmp=log(mp)
gen lg=log(g)
spatwmat using W.dta,name(W) standardize
xtset city year
xsmle lgdp ls ld ll lk lg lfdi lgov lmp,wmat(W)model(sdm)robust nolog noeffects
stata运行结果如下：