Can't download document when calling function getDocument

Error: Call to a member function read() on string in DocuSign\eSign\ObjectSerializer::deserialize() (line 285 of/vendor/docusign/esign-client/src/ObjectSerializer.php).

I'm calling that function

$temp_file = $envelope_api->getDocument($accountID, $document_id, $envelope_id);

This is causing the issue

$file = fopen($filename, 'w');
while ($chunk = $data->read(200)) {
    fwrite($file, $chunk);
}
fclose($file);