Optional elements in SOAP Envelope

Hi.

I'm getting the following response from a remote service:

<?xml version="1.0" encoding="UTF-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
 <SOAP-ENV:Header xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>
 <S:Body xmlns:S="http://schemas.xmlsoap.org/soap/envelope/">
  <payload></payload>
 </S:Body>
 <moreData xmlns="someNamespace">
 </moreData>
</soapenv:Envelope>

where "moreData" represents some optional node.
This is a valid SOAP message, but the "SOAP Response" assertion in SOAPUi keeps failing with the message "Element not allowed: moreData@someNamespace in Element Envelope@http://schemas.xmlsoap.org/soap/envelope/"

Please, does anybody knows how is SOAPUi validating the "SOAP Response" assertion?.

Thanks.