I got Invalid arguments in an Particle function, and dont know why.

Im using A6 v.6.60

Code:

function rocket_smoke()
{
	while (1)

       {

               effect(rocket_smoke_behavior() ,50, my.x, normal);
               
        }
}


function rocket_smoke_behavior()
{
		my.bmap = pcxSmokeParticel;
		
		my.TRANSPARENT = on;
	
		 my.alpha = 60 ;
			my.lifespan = 80;
       my.flare = on;

       my.bright = on;

       my.move = on;

       my.function = fade_smoke();


}


function fade_smoke()
{
	
	my.TRANSPARENT = on;
	
	
		
		my.alpha -= 1.2 * time_step;
      if (my.alpha <= 0) { my.lifespan = 0; } 

        


}


The invalid argument is in function fade_smoke

INVALID ARGUMENT : (my.alpha <= 0)

Please, anyone may help me.

I took the code from AUM, and from manual, i dont know why this is an invalid argument.

Thanks to all

Mike