imagecolorsforindex（）：将动画gif转换为jpg时，颜色索引244超出范围

I'm using Laravel and the SimpleImage class to convert images to thumbnails.

this is the code that converts the images and produces the error:

    $img = new SimpleImage(public_path() . '/img/temp/' . $rnd . '.' . $ext);
    $img->fit_to_width(125)->save(public_path().'/img/thumbs/'.$rnd.'.jpg');
    unlink(public_path().'/img/temp/'.$rnd.'.'.$ext);

the variables $rnd and $ext are assigned before this code. Also, not all gifs produce this error, some work, some don't.

I can't figure it out..