IndexError: array index out of range in next()

[amit.kehat]

Hello,

I am facing the next issue.
This is my code:

class ShortSuperTrend(bt.Indicator):
    lines = ('ShortSuperTrendUp',)

    params = (('atr_period', 14), ('Multiplier', 0.6), )

    def __init__(self):
      self.atr = bt.talib.ATR(self.datas[0].high, self.datas[0].low, self.datas[0].close, timeperiod=self.params.atr_period)
      self.up = self.datas[0].low - (self.params.Multiplier * self.atr)
      self.up1 = self.up(-1)
      #self.lines.ShortSuperTrendUp = self.up1
         
    def next(self):
      self.up = bt.If(self.datas[0].close[-1] > self.up, bt.Max(self.up, self.up1), self.up[0])

When using indexing with () in the __init__() I am getting the next error:

TypeError: 'list' object is not callable

The assignment in the next() function produce the next error:

IndexError: array index out of range

My wish is to update self.up with the expression's results for each candle.