I have a problem concerning a landing page and 404 errors.

**Context :** I have a content type "news", with auto url /news/my-news-title. I have created programmatically a kind of homepage for news, with /news as URL


**What's wrong :** My problem is that if I have a link like /news/aiejaziejfnjdfnj (it doesn't exist), I don't have a 404 error and this is my /news landing page which is shown.
What I wish, with you help, is to have a 404 error when I have a non existant URL.

I'm going to show you what I have done, and hope sincerely you'll have an idea of what i'm doing wrong. In a custom module, I have :

    function mymodule_menu(){
        $items = array();
        $items['news'] = array(
           'title' => "Les news",
           'page callback' => 'landing_page',
           'access arguments' => array('access content'),
        );
        return $items;
    }
    function landing_page(){
        return "nothing";
    }

Then, I have a template page--news.tpl.php in which I have write things manually without showing $content (the "nothing").

So, why /news/aizejziej is accepted and is showing /news ? Is it function landing_page() which is doing something bad by always returning something ?

Thanks in advance for your help/ideas :)