Error SOAP:1.001 CX_ST_GROUP_MISSING_CASE while calling a method

Hello,

After having generated an ABAP proxy consuming class for a SOAP Web Service, I'm calling one of the methods in a program but the resulting return is empty. So I've tested the same method in SE80 to find out what's going wrong and I'm getting the following error:

SOAP:1.001 CX_ST_GROUP_MISSING_CASE:.Element {http://www.indexeducation.com/frahtm/HpSvcW.html}return missing.

Here is the complete answer from the call:

<n0:NombreCoursResponse xmlns:n0="http://www.indexeducation.com/frahtm/HpSvcW.html" xmlns:NS1="http://www.indexeducation.com/frahtm/HpSvcW.html"xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns:xsd="http://www.w3.org/2001/XMLSchema"xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"><return xsi:type="xsd:unsignedInt">15545</return></n0:NombreCoursResponse>

the message seems correct, I think I've missed something during the class generation but can't find what.

Thanks