Primary key S_COR_ID-VALUE is not available

I am working on Gateway Service using RFC Gen.facing an issue while creating the Data Model. I have selected BAPI_USER_GETLIST. Next I have clicked on 'create mapping'. I have chosen operation type as 'Query'. I have set the constant values for MAX_ROWS and WITH_USERNAME (as per the directions given in the step-by-step procedure by SAP).Now when I am clicking on the u2018Change Mapping Routeu2019 button for the USERNAME attribute within the USERLIST node,the prmary key S_COR_ID-VALUE is not showing up in the pop-up window as it ideally should have. The other options are showing up e.g. TYPE,ID,NUMBER,MESSAGE etc. I am working with Netweaver Gateway Service Pack 3.0. What might be the possible reason behind this problem?