[Plugin: Photon] Use photon with featured images

Hi all,

I’m trying to use photon in a featured image. This is the code:

$post_thumbnail_src = wp_get_attachment_image_src( get_post_thumbnail_id(), 'large' );
echo '<img src="'. apply_filters( 'jetpack_photon_url', $post_thumbnail_src[0] , 'filter=grayscale' ) .'">';

All goes well with photon deactivated, but when active I get this in the frontend:

<img src="/?filter=grayscale=https://i0.wp.com/mysite.com/wp-content/uploads/2015/10/bike-and-guy.jpeg?fit=1200%2C9999" />

The filter is being prepended to the url instead of being appended as a query string parameter.
What am I doing wrong?

Thanks in advance,