Error: Serializer not found

Hi all,

         I am new in working with web service. I am using WSDK5.1 server to
deploy the web services and using WSIF API to invoke them. The web service
just takes a String and returns a String, but in my java client I am getting
this error.
java.lang.IllegalArgumentException: No Serializer found to serialize a
'java.lang.String' using encoding style 'literal'.
Can anybody help me in this regard. Thanks a lot for your help. 

Thanks
Ravi