Unable to create a shipment - missing Collection Date

[mikemike]

The API is always telling me that I'm missing a Collection Date, the exact error:

SoapFault
Missing Collection Date

This is my code:

$params = [
    "ServiceID" => "9991",
    "ClientReference" => "123",
    "CollectionDate" => "2021-04-01",
    "Parcels" => [
        [
            "Weight" => 5,
            "Length" => 5,
            "Width" => 5,
            "Height" => 5,
            "Contents" => "Box",
            "Value" => "9",
        ],[
            "Weight" => 5,
            "Length" => 5,
            "Width" => 5,
            "Height" => 5,
            "Contents" => "Box",
            "Value" => "9",
        ]
    ],
    "RecipientAddress" => [
        "RecipientName" => "Michael Griffiths",
        "RecipientAddress" => [
            "Street" => "Redacted",
            "Locality" => "Redacted",
            "TownCity" => "Redacted",
            "County" => "Redacted",
            "PostalCode" => "Redacted Redacted",
            "CountryCode" => "GB",
            "RecipientPhone" => "07Redacted",
        ],
        "RecipientEmail" => 'Redacted@Redacted.com',
    ]
];

return $this->client->AddShipment($params);

The date I'm passing (tomorrow) is available in the returned array from GetAvailableCollectionDates using the same address. I have tried other dates also in the GetAvailableCollectionDates array but no matter what I try I get the same error. The ServiceID is from the GetAvailableServices method.

I'm using a demo account.

What am I missing?