Får Error 500 av GD imagecopy i apache php

$gradient = imagecreatetruecolor($scale, $scale);
for ($i = 0; $i < $scale; $i++) {
	$alpha = round((127 / $scale) * $i);
	$color = imagecolorallocatealpha($gradient, 0, 255, 0, $alpha);	
	imagefilledrectangle($gradient, $i, 0, ($i + 1), $scale, $color);
}