Cannot change template of document?

        $document   = $this->modx->getObject('modDocument', $id);
      
        $document->set('content', $newContent);
        $document->set('template', 0); // also tried null and 1
        if(!$document->save()) {
            $this->modx->log(modX::LOG_LEVEL_ERROR, 'An error occurred while saving document.');
        }

$resource-setContent($newContent);