fill background color between 2 lines.new in pinescript

Question

I am trying to fill the background color between these 2 lines and I am getting the following error messages:

line 104: Syntax error at input 'bullEngulfLow'.

I am using the following code.

if barstate.islastconfirmedhistory
    fillColor = switch
    bullEngulfOpen > bullEngulfLow => color.green
    => color.silver
linefill.new(bullEngulfOpen, bullEngulfLow, color.new(fillColor, 90))

bullEngulfOpen := line.new(bar_index - 1, resopen[1], bar_index, resopen[1], extend=extend.right, color=color.black)
bullEngulfLow := line.new(bar_index - 1, bullEngulfLowPrice, bar_index, bullEngulfLowPrice, extend=extend.right, color=#01ff00)

How do I fill the background color between these 2 lines?