This issue comes into picture whenever there is any issue at WCF service.

So see more details about the issue, you need to includeExceptionDetailInFaults attribute to TRUE in serviceDebug tag.

<serviceBehaviors>
<behavior name="myServiceBehavior" >
 <serviceDebug includeExceptionDetailInFaults="true" />
 </behavior>
</serviceBehaviors>

Either you can also add class attribute to the service class

Happy coding.. Hope this helps!