How to verify a pdf is signed?

Hello =)

I'm working with LiveCycle ES4 designing a form and I want my form to be filled by Adobe Reader users.

In this form, I have a signature field which I want to verify before the user submits the document.

I'm using this code in the click event of the button:

     var signature = event.target.getField("Form.subForm.signature");

     var signatureState = signature.signatureValidate();

     if (signatureState == 0){

          xfa.host.messageBox("Please sign the document.");

          }

     else if (signatureState == 1 || signatureState == 2){

          xfa.host.messageBox("Please use a valid signature.");

          }

     else if (signatureState >= 3){

          event.target.submitForm({cUrl:"mailto:xxxxxxxx@xxxx.xxx", cSubmitAs:"PDF"});

          xfa.host.messageBox("Form submited.");

          }

I've extended the features of this form in Reader using Acrobat Pro Extended.

The problem is..this line:

     var signature = event.target.getField("Form.subForm.signature");

.. isn't working the way it's supposed to. When I press the button in the signed, or unsigned, document, this variable is null.

Can PLEASE anyone explain me why?