Trying to mock SOAP service, but get error .. Encountered illegal extension attribute 'nillable'

Trying to generate a mock SOAP service, but I get an error ...

WSDLException (at /definitions/message[23]/part[1]): faultCode=INVALID_WSDL: Encountered illegal extension attribute 'nillable'. Extension attributes must be in a namespace other than WSDL's

which looks to be because of the element ...

<part name="ChangesSince" nillable="true" type="xs:dateTime" />

... certainly if I remove it, SoapUI stops complaining and generates a mock service 

But if I then generate WCF clients against the mock service, the date time is a ...

private System.DateTime closureDateField;

whereas against the real web service they generate as ...

        [System.Xml.Serialization.SoapElementAttribute(IsNullable=true)]
        public System.Nullable<System.DateTime> ClosureDate {

Is there some way to handle nillable's in SoapUI?