Getting 'undefined' as well as expected results - why?

I've got an aftersubmit running on my kit items to grab the internal id of the kit members and populating the results to a custom field. The results are always prefaced with 'undefined' - any idea why?

Code:
function item_save ()
{
var lines, line, items, itemname = [], desc;
lines = nlapiGetLineItemCount('member');
for(line=1;line<=lines;line++)
{
itemname = nlapiGetLineItemValue('member','item',line);
if (itemname.length != -1)
{
items.push(itemname);
}
}
desc += items.join(' ');
nlapiSetFieldValue('custitem31',desc);
return true;
}


The result in the custom field is:
undefined15714 21838 20037 22251

There are only 4 members of this kit. So the 4 internal ids returning are corrct. Just a damn annoying "undefined" at the start that I can't get rid of.