Validation on image_advanced

Hi All,

I want to ask how to set validation on image_advanced fields? I try to set ‘required’ validation on image_advanced, when there is no image, it says ‘The field is required’, but when I choose an image, it keeps saying ‘The field is required’, and the form cannot be submitted. The metabox code is as below :

$meta_boxes[] = array(
	'id'         => 'about',
	'title'      => __( 'About Us', 'meta-box' ),
	'post_types' => array( 'about_us' ),
	'context'    => 'normal',
	'priority'   => 'high',
	'autosave'   => true,
	'fields'     => array(
		array(
			'name'  => __( 'Banner Image', 'meta-box' ),
			'id'    => "image_banner",
			'type'  => 'image_advanced',
			'max_file_uploads' => 1
		)
	),
	'validation' => array(
		'rules'    => array(
			"image_banner" => array(
				'required'  => true
			)
		),
		'messages' => array(
			"image_banner" => array(
				'required'  => __( 'Banner Image is required', 'meta-box' ),
			)
		)
	)
);

Thank you.