Error: property_groups: expected list, got string

I've been struggling getting my properties to show. I created a template fine, assigned a property to an item, and now I want to view a folder list with items + see their properties. I get this error: 

"Error in call to API function "files/list_folder": request body: include_property_groups.filter_some: expected list, got string".

Here is my code below. What format is expecting for "include_property_group"?

    $meta = array(
        ".tag" => "filter_some",
        "filter_some" => "ptid:-ynhqttM3GkAAAAAAAAeOg"
    );


    $fields = array(
        "path" => "/folder 2",
        "include_property_groups" => $meta,
        "recursive" => false,
        "include_media_info" => false,
        "include_deleted" => false,
        "include_has_explicit_shared_members" => false,
        "include_mounted_folders" => false
    );

    $headers = array(
        'Authorization: Bearer ' . $auth_token,
        'Content-Type: application/json'
    );

    $ch = curl_init();
    curl_setopt( $ch, CURLOPT_POSTFIELDS, json_encode( $fields ) );
    curl_setopt( $ch, CURLOPT_RETURNTRANSFER, true );
    curl_setopt( $ch, CURLOPT_CUSTOMREQUEST, "POST" );
    curl_setopt( $ch, CURLOPT_HTTPHEADER, $headers );
    curl_setopt( $ch, CURLOPT_URL, 'https://api.dropboxapi.com/2/files/list_folder' );