Getting error while calling the .svc Web Service using soapU

I'm calling the .svc?wsdl file from SoapUIPro, it shows me the error

<s:Text xml:lang="en-US">Error in deserializing body of request message for operation 'subscribe'.</s:Text>

<Type>System.ServiceModel.CommunicationException</Type>

HTTP/1.1 500 Internal Server Error
Date: Mon, 14 Nov 2011 09:22:11 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
Cache-Control: private
Content-Type: application/soap+xml; charset=utf-8
Content-Length: 755


Any guidance will be appreciated. Why this is happening?
Thanks