Web service error: WS Security can only be applied on a SOAP document

Hello,

I am facing the problem that when calling a web service I got this error message:

005056B855B2002A0000013B000032C70005105EF7348C38 : WS Security can only be applied on a SOAP document.

Some information about the webservice:

Transport Binding: HTTP SOAP with Attachments

Authentication: Basic

Netweaver version: 7.01.06

The webservice is based on EJB. Basically I want to add two operations in the webservice: one to accept attachment as input. the other one returns a file as attachment.

Does anyone knows why this happens and how to resolve it? Thanks in advance!

Regards,

Grant