Can't add a photo into a gallery

[mardy]

Hi again :-)
I'm calling the method PUT users/:user_id/galleries/:id/items to add a photo into a gallery (precisely, the url I'm using is https://api.500px.com/v1/users/22027935/galleries/29303601/items), with this body:

{
  "add": {
    "photos": [238526019]
  }
}

The method returns HTTP 200 (it does it all the time, even if the passed JSON is invalid!), with an empty object {} and indeed does not add the photos to the gallery.

I've also tried calling this method from the API console and it behaves in the same way.