php - Illuminate中的call_user_func出错

/**
 * Get the current PDO connection.
 *
 * @return \PDO
 */
public function getPdo()
{
    if ($this->pdo instanceof Closure) {
        return $this->pdo = call_user_func($this->pdo);
    }

    return $this->pdo;
}


为什么call_user_func会出错呢？