Undefined variable color.inc

line 48 in color.inc of the compass module

function sassy_compass__ie_hex_str($color) {
	SassLiteral::assertType($color,  'SassColour');
	$alpha = round($color->alpha * 255);
	$alpha_str = str_pad(dechex($alpha), 2, '0', STR_PAD_LEFT);
	$col = $color->asHex(FALSE);
	return new SassString(strtoupper("##{$alphastr}#{$col}"));
}

$alphastr is undefined.