I seem to be having a problem with the alpha setting of a sprite that I create. When I use the code below the line is completely invisible, if I set the alpha to 55 it is visible. Can anyone help me see why I can't make it lower?

bmap dot_bmp ="dot.bmp";

function delete_line()
{
my.alpha - = 5*time;
if(my.alpha < = 0)
{
my.lifespan = 0;
}
}

function draw_red_line()
{
my.bmap = dot_bmp;
my.move = off;
my.streak = on;
my.bright = off;
my.transparent = on;
my.alpha = 50;
my.size = 0.25;
my.function = delete_line;
}