IF ELSE statement with && and || condition

I have a group of checkboxes (id = "first" id = "second") and the main checkbox (id = "main").

<input type='checkbox' id="main_button" onclick="Indeterminate()"><label for="main_button">Main checkbox of group</label> 
<input type='checkbox' id="first" onclick="Indeterminate()"><label for="first">First thing</label>
<input type='checkbox' id="second" onclick="Indeterminate()"><label for="second">Second thing</label>

If one or more of the group checkbox checked then the main have indeterminate condition. If all checked then the main checkbox have also checked condition.

function Indeterminate() {
    if (document.getElementById('first').checked || document.getElementById('second').checked) {
        document.getElementById('main_button').indeterminate = true;
    } else if (document.getElementById('first').checked && document.getElementById('second').checked) {
        document.getElementById('main_button').checked;
    } else { 
        document.getElementById('main_button').indeterminate = false; 
    }    
}

In my IF ELSE statement, conditions IF and ELSE works, but there is something wrong with ELSE IF. Probably doing a simple mistake or? Thank you!

JSFiddle example