soapUI mock service - escape "&&" in request

Hi!
I'm working on mock service and cannot find the resolution for this issue.
Service is called with random data and sometimes "&" appears in the input string. Mock considers it as an entity (not ended with ";") and returns SOAP Fault:

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope">
   <soap:Body>
      <soap:Fault>
         <soap:Code>
            <soap:Value>Server</soap:Value>
         </soap:Code>
         <soap:Reason>
            <!--1 or more repetitions:-->
            <soap:Text xml:lang="en">org.apache.xmlbeans.XmlException: error: Unexpected element: CDATA</soap:Text>       </soap:Reason>  
      </soap:Fault>
   </soap:Body>
</soap:Envelope>

Is there any way to bypass it?
I have tried to use OnRequest script to filter the ampersand / translate it into entity, but failed.
It seems that the validation is made before the script is run.

Thanks,
Michal