Expand subtabs in suitelet

Hi, we have a requirement to display 2 sublists in "unlayered mode". Clicking on "Expand Tabs" generates a message:"This request is missing a required parameter". This happens only during a POST write from the suitelet.

 function multitabbedSublist(request, response){   if (request.getMethod() == 'GET') {    var form = nlapiCreateForm('GET - Use Subtabs and Expand Tabs is working');    form.addSubmitButton('submit');    response.writePage( form );   }else{    var form = nlapiCreateForm('POST Use Subtabs and expand tabs does not work');    var custpage_sublist1 = form.addSubList('custpage_sublist1', 'list', 'Company','main');      custpage_sublist1.addField('custpage_companyname', 'text', 'Name');      custpage_sublist1.addField('custpage_phone', 'text', 'Phone');    custpage_sublist1.setLineItemValue('custpage_companyname',1,'Company 1');    custpage_sublist1.setLineItemValue('custpage_phone',1,'1234567');    custpage_sublist1.setLineItemValue('custpage_companyname',2,'Another Company');    custpage_sublist1.setLineItemValue('custpage_phone',2,'8765432');    var custpage_sublist2 = form.addSubList('custpage_sublist2', 'list', 'Contacts','main');      custpage_sublist2.addField('custpage_firstname', 'text', 'First Name');      custpage_sublist2.addField('custpage_lastname', 'text', 'Last Name');    custpage_sublist2.setLineItemValue('custpage_firstname',1,'My');    custpage_sublist2.setLineItemValue('custpage_lastname',1,'Name');    custpage_sublist2.setLineItemValue('custpage_firstname',2,'Another');    custpage_sublist2.setLineItemValue('custpage_lastname',3,'Name');    form.addSubmitButton('submit');    response.writePage( form );   }  }