Checkbox Error - Message Elements Undefined

Firebug returns an error saying msg.elements is undefined. I'm trying to make a function that will enabled a delete button if any of the check boxes in a list are checked.

function selectone () {
var msg = document.getElementsByName('pm');
var i = 0;
for(i; i < msg.elements.length; i++){
if(msg.elements[i].checked == true){
document.getElementById('multiple_action').disabled = false;
document.getElementById('drop_button').setAttribute("class", "drop_button");
}else{
document.getElementById('multiple_action').disabled = true;
document.getElementById('drop_button').setAttribute("class", "drop_button disabled");
}}}