Validate Required Multiselect And Checkbox On Form?

I need to do a clientside check of a form to see if a "required" multiselect list and a checkbox have been left blank or not. To clarify, the user is required to select either one or more options from the multiselect list, the checkbox, or a combination of both.

What they can't do is leave both of them blank. Both the multiselect and the checkbox pass values to a PHPList script on the server as if they were one multiselect with the name "attribute17[]".

***Disclaimer - I am a total javascript noob and am piecing together examples from several days (and long nights) of googling.***

I'm dealing with a very long subscribe form, and so far I've been successful with figuring out the code to validate emails, select dropdowns, and checkboxes. This combination multiselect and checkbox is the last piece of the form that is giving me trouble. Hopefully somebody here has a solution.

For simplicity I've reduced the javascript and the form down to just the part I'm still having trouble with. Here is a piece of the javascript that I'm trying to use:

Code:
<script language="Javascript" type="text/javascript">
function checkform() {
if(document.subscribeform.elements["attribute17[]"].value == "")
{
alert("Please select your Primary Work Locations");

[Code]...