Natr index out of range

[wzy2687]

data =[]float64 {1,2,3}
Natr(data,data,data,3)

func Natr(....).. {
...
prevATRTemp := Sma(tr, inTimePeriod)
prevATR := prevATRTemp[inTimePeriod]   // will panic 
...
}

[wzy2687]

data err.