Require signature field to be signed before submit

Hello,

I have created a form that gathers information and needs to require a Digital signature before it can be submitted.  I am using livecycle 8.2 and will use the Distribute tool to distribute and track and gather responses.  I have used the following script in the preSubmit event to make several required fields mandatory before they can click the adobe-created Submit button in the distributed form:

if(this.isNull) 
{
this.mandatory = "error";
xfa.host.messageBox("Enter information in the Name field."); 
}   
else
{
this.mandatory = "disabled";
}

I have been unable to require the Signature field with this same script.  I placed the script above in the preSubmit event for my Signature field however, if a user does not sign the document, and in turn then receives the message box and mandatory error, when they do sign the document they still continue to receive the message box and mandatory error.  It appears that a Signature field's rawValue is always null?  How do I make the form recognize when the Signature field is signed?

Your help is greatly appreciated,

Tilson