利用NewMagickWand生成gif的问题

$array = array("X02.jpg","X03.jpg");
$mw = NewMagickWand();
for($i=0; $i<count($array);$i++) {
    $rw = NewMagickWand();
    MagickReadImage($rw,$array[$i]);
    MagickSetImageDelay($rw,300);//300=3秒
    MagickAddImage($mw,$rw);
    DestroyMagickWand($rw);
}
MagickSetFormat($mw,"gif");
MagickWriteImages($mw,"animate.gif",MagickTrue);
$mw = DestroyMagickWand($mw);

利用NewMagickWand生成gif，报

Fatal error: magickaddimage(): C API unable to retrieve the current active image of the 2nd MagickWand resource argument; unable to perform the copy operation (reason: wand contains no images `MagickWand-40' @ error/magick-image.c/MagickGetImage/3848) [on C source line 3750] in /opt/app/phpapp/devplatform/test/image_m.php on line 8

这个错误是怎么回事啊？

貌似错误出在这一行——  DestroyMagickWand($rw);