This is one of the silly problem that does not provide any clue. The only message received at client while invoking service is “Object reference not set to an instance of an object”. If you debug the client and everything looks good, then the problem should be at Server.
- Turn on the WCF service trace at Server. Check the service log.
- Run Fiddler at Client and capture the SOAP trace. Check the log.
You should be able to see the error SOAP message by performing one of the two. Go ahead and fix it.