用purtest函数进行面板数据单位根检验出现这种情况是什么意思，p值为啥没有
> purtest(object=pdata[,8],exo="intercept",test="ips",lags="AIC",pmax=4)

        Im-Pesaran-Shin Unit-Root Test (ex. var.: Individual Intercepts)

data:  pdata[, 8]
Wtbar = NaN, p-value = NA
alternative hypothesis: stationarity

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