Returning multiple itemType=playlist mediaCollections when asked for getMetadata id=playlists only shows one playlist

Sonos sends me this query (abridged):

<getMetadata>
  <id>playlists</id>
</getMetadata>

I respond:

<getMetadataResponse>
    <getMetadataResult>
        <count>2</count>
        <index>0</index>
        <total>2</total>
        <mediaCollection readOnly=\"false\" userContent=\"false\" renameable=\"false\">
            <itemType>playlist</itemType>
            <id>playlist:29986647</id>
            <title>playlist-1</title>
            <albumArtURI>uri1</albumArtURI>
            <canPlay>true</canPlay>
        </mediaCollection>
        <mediaCollection readOnly=\"false\" userContent=\"false\" renameable=\"false\">
            <itemType>playlist</itemType>
            <id>playlist:28002250</id>
            <title>playlist-2</title>
            <albumArtURI>uri2</albumArtURI>
            <canPlay>true</canPlay>
        </mediaCollection>
    </getMetadataResult>
</getMetadataResponse>

In the Sonos app (S2), when I click on Playlists it just shows the title of the page as "playlist-1" and no tracks are listed. playlist-2 is ignored, and I don't get a call to return the tracks for playlist-1.

I guess this is something to do with mediaCollections, itemTypes and containment - should there be a root collection? Am I using the correct itemType?

Regarding itemType, I noticed that when displaying genres, these seem to work when returned as albumLists rather than genres. I have tried trackList as an alternative to playlist but that doesn't work.

The code I'm using is based on bonob.

I found that the problem was that when getMetadata was called for the root collection, we were returning the playlists container with itemType as playlist . This meant the resulting item, when clicked, wasn't drawn as a container. Changing this to container fixed the problem and showed all the playlists.