Don't assume. Trace the call and look into it via "svctraceviewer.exe".
Setup trace by including something like:

<system.diagnostics>
   <sources>
     <source name="System.ServiceModel" switchValue="Verbose,
ActivityTracing" propagateActivity="true">
       <listeners>
         <add name="xml" />
       </listeners>
     </source>
     <source name="System.ServiceModel.MessageLogging" switchValue="All">
       <listeners>
         <add name="xml" />
       </listeners>
     </source>
   </sources>
   <sharedListeners>
     <add
       name="xml"
       type="System.Diagnostics.XmlWriterTraceListener"
       initializeData="c:\temp\trace.svclog" />
   </sharedListeners>
 </system.diagnostics>

Tiago Halm