argument is nil inside function in table

monitor={
    left_margin=3,
    top_margin=3,
    line_height=7,
    lines={},

    new=function(self,tbl)
        tbl=tbl or {}
        setmetatable(tbl,{
            __index=self
        })
        return tbl
    end,

    add_line=function(self,lin)
    printh("in add_line, lin="..lin,"bugfile.txt")
        add(self.lines,lin)
    end,

    draw=function(self)
        for i=0,#self.lines-1 do
            print(self.lines[i+1],left_margin,top_margin+i*line_height,7)
        end
    end
}

eoum=monitor:new()

eoum.add_line("finalbx:"..bl.x)

eoum:draw()