SOAP Fault message replaced by IIS error

Hi All,

I have hosted a JAX-WS web service, (developed on websphere application server) on IIS server.

Whenever a request xml in the soapMessage body is invalid, we construct a piece of custom XML with the error details and return this in the soapFault detail.

However, it seems that IIS is intercepting and replacing our custom soapFault message with a "pretty" html page(500 Internal server error )

Does anybody know how I can stop this from happening?