Call to a member function toArray() on array

[TonyMarston]

Problem description:

Error:

Uncaught exception from Error, message = Call to a member function toArray() on array, code=0, file=F:\Apache2\HTDOCS\GMX-Transix\includes\vendor\jlevers\selling-partner-api\src\Traits\HasArrayableAttributes.php, line=68

Code

require_once('vendor/autoload.php');
use SellingPartnerApi\SellingPartnerApi;
use SellingPartnerApi\Enums\Endpoint;
use SellingPartnerApi\Seller\APlusContentV20201101\Dto;

$connector = SellingPartnerApi::seller(
    clientId: $client_id,
    clientSecret: $client_secret,
    refreshToken: $refresh_token,
    endpoint: Endpoint::{$endpoint}
);

$api = $connector->aPlusContentV20201101();

try {
    $result = $api->validateContentDocumentAsinRelations(
        marketplaceId: $marketplace_id,
        postContentDocumentRequest: new Dto\PostContentDocumentRequest(new Dto\ContentDocument($name,
                                                                                               $contentType,
                                                                                               $locale,
                                                                                               $contentModuleList,
                                                                                               $contentSubType)));
} catch (Exception $e) {
    echo 'Exception when calling aPlusContentV20201101->validateContentDocumentAsinRelations: ',
        $e->getMessage(),
        PHP_EOL;
};

The code appears to fail when processing the $contentModuleList argument, but as this is already an array why is it failing?