GD Adaptor Image resize & crop

I'm triying to resize and crop an image with the Phalcon\Image\Adapter\GD

The docs says

Resize the image to the given size. Either the width or the height can be omitted and the image will be resized proportionally.

but I tried with null and 0 on the first parameter and an exception is throw:

width and height must be specified


only resize allows null as a parameters, is that how you use it ?

can we see the code ?

edited Sep '15

    public function testsAction()
        try {
            $image = new Phalcon\Image\Adapter\Gd('absolute_path_on_server');
            $image->resize(null, 800);
        } catch (\Exception $e) {

throws width and height must be specified

$image->resize(null, 800);

You're trying to resize to Width = null, so it's throwing you the error. You need 2 values for your resize function adn they can't be null values : _resize (mixed $width, mixed $height)