Symfony 1.4在自定义过滤器中进行数据库操作后，异常不起作用

this question might looks a bit strange. Lets see this filter:

public function execute ($filterChain)
{
    if ($this->isFirstCall())
    {
        throw new Exception ('fail');
    }

    $filterChain->execute();
}

well this works just fine. But I need a database operation:

public function execute ($filterChain)
{
    if ($this->isFirstCall())
    {
        xTable::getInstance()->find (1);
        throw new Exception ('fail');
    }

    $filterChain->execute();
}

after using this query: xTable::getInstance()->find (1); Symfony 1.4 forgets to show exceptions: only a white page is returned. The query itself ran ok! This way is impossible the debugging - all I got is white pages then. What can be the error?