I try to understand something regarding getelementbyid.

here is a stupid test of Javascript.

background-color : #BBBBBB;
/*width : 200px;*/
/*height : 200px;*/
background-color : #DD80DD;


<img src='A_des_img/node.gif' style='vertical-align:middle;background-color:#EEAAFF;'>
<b onclick='alert(document.getElementByID("id_maindiv").id);' id='b_id' style='vertical-align : middle;background-color:#EEAAFF;'>my 1st div</b><br>

<div class='maindiv' id='id_maindiv'>


when i click on <b> tag, i should display an alert message with "id_maindiv" as text.
however, nothing is displayed, why ?
Maybe i'm very tired and i do not have well opened eyes, but i would like to understand...

thanks a lot,