擦除输入字段后，javascript将无法读取它

I have a simple function for fetching a result,

<script language="javascript">
        function searching(value){
            url="ajax_search.php?st=usu&sear="+value;
            ajax(url);
        }
        searching(" ");
</script>

And my html:

<input type="text" id="usu" value="" name="usu" style="width:250px;" onKeyUP="searching(this.value)" />

My problem is that, if you write, for example= "test" on the input field, it will search and show all the matches for "test", if you erase that and search for "example" it wouldn't do anything. Like if you haven't typed anything.

Can someone help me?

EDIT: The console says:

http://pesquisa.mqb.com.br/relatorios/pesq/ajax_search.php?st=usu&search=TEST Failed to load resource: the server responded with a status of 403 (Forbidden)

Found the problem, my firewall was blocking due to many connections try in a short space time as I press backspace much faster than I type. Thank you for commenting!

You may wanna modify onKeyUp to onkeyup

<input type="text" id="usu" value="" name="usu" style="width:250px;" onkeyup="searching(this.value)" />

Also, print the value you get in the log:

function searching(value){
    url = "ajax_search.php?st=usu&sear="+value;
    ajax(url);
    console.log(value);
}