WP REST API: _fields not returning featured image

I have used a function to register rest field as below:

add_action('rest_api_init', 'register_rest_images' );
function register_rest_images(){
    register_rest_field( 'post',
        'fimg_url',
        array(
            'get_callback'    => 'get_rest_featured_image',
            'update_callback' => null,
            'schema'          => null,
        )
    );
}
function get_rest_featured_image( $object, $field_name, $request ) {

    if( $object['featured_media'] ){
        $img = wp_get_attachment_image_src( $object['featured_media'], 'thumbnail' );
        return $img[0];
    } else {
		return false;
	}

}

when I check this- https://mysite.local/wp-json/wp/v2/posts
it returns fimg_url with the image url, but when I try this –
https://mysite.local/wp-json/wp/v2/posts?_fields=fimg_url
it always returs false

Am I using this in a wrong way? I have used some other fields other that using featured image, that works. So any advices will be helpful.