ALBUM_SEARCH doesn't return coverart

I have the following issue with the Web API. When I perform a query type ALBUM_SEARCH with the option SELECT_EXTENDED=COVER it doesn't return coverart. Using <MODE>SINGLE_BEST_COVER<\MODE> I do get the cover url.

Below are the options I tried adding to the query. The first one returns no url's, the second one returns review, biography and image url's but not the cover url. Why I don't get the cover url's ?

<option>
<parameter>SELECT_EXTENDED</parameter>
<value>COVER</value>
</option>

<option>
<parameter>SELECT_EXTENDED</parameter>
<value>COVER,LINK,REVIEW,ARTIST_BIOGRAPHY,ARTIST_IMAGE</value>
</option>

Kind regards, Marc