flat log likelihood encountered, cannot find uphill direction

set more off       
qui arima sale time if code== 1001 & time >= tm(2009-05) & time <= tm(2012-08), ar(3) ma(3)       
estat ic

我用40个月的时间，来预测。   想提取AIC 和BIC出来，不行，不知道为什么，出现这段报错文字
flat log likelihood encountered, cannot find uphill direction

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