IssuePicker.jspa JS error

function populateFormMultiple()
{
    var issuelist = '';

    if (document.issues.issuekey.length > 0)
    {
        // get all the selected checkboxes
        for (var counter=0; counter<document.issues.issuekey.length; counter++)
        {
            if (document.issues.issuekey[counter].checked == true)
            {
                if (issuelist == '')
                {
                    issuelist = document.issues.issuekey[counter].value;
                }
                else
                {
                    issuelist = issuelist + ', ' + document.issues.issuekey[counter].value;
                }
            }
        }
    }
    else if (document.issues.issuekey.checked == true)
    {
       issuelist = document.issues.issuekey.value;
    }
    var openerForm = window.opener.document.forms['i-mark-as-dependent-form'];
    var openerLinkEl = openerForm.elements['i-link-widthout-edit-input'];
    openerLinkEl.value = issuelist;
    window.close();
}