Getting Empty Rows in Google Analytics Data API

Having difficulty getting empty rows from the Analytics Data API (GA4) BETA. I'm pulling simple traffic by date array but any dates that have zero traffic are not returned - I would prefer it to give me the date and zero.

I believe there should be a KEEPEMPTYROWS parameter but this doesn't seem to work - it took me so long to get the ORDERBY working that I thought I'd check here first to see if anyone else has got this working lol. I know I can fill in the missing dates using PHP code but I'd prefer to get it from the query. Here's the PHP for my JSON query:

$response = $client->runReport([
'property' => 'properties/' . $_SESSION["gaid"],
'dateRanges' => [
    new DateRange([
        'start_date' => $mstartdate,
        'end_date' => $menddate,
    ]),
],
'dimensions' => [new Dimension(
    [
        'name' => 'date',
    ]
),
],
'metrics' => [new Metric(
    [
        'name' => 'activeUsers',
        'name' => 'sessions',
    ]
)
],
'orderBys' => [
    new OrderBy([
        'dimension' => new OrderBy\DimensionOrderBy([
            'dimension_name' => 'date',
            'order_type' => OrderBy\DimensionOrderBy\OrderType::ALPHANUMERIC
        ]),
        'desc' => false,
    ])],
    'keepEmptyRows'=>"1"

]);

Thanks in advance :)