Error Variable type does not match the operation message type

Hi there, when I try to use Oracle JDeveloper 10g to develop a Business Process Execution Language BPEL business process with web services, an error appears in the reply activity. The validation error says that variable type and operation message type are different and the former does not match the latter.
Could anyone advise the solution please? Thanks in advance.

Error Variable type does not match the operation message type.
