sys_context inside a packaged procedure?

I'm trying to retrieve a user's current schema from within a procedure but all I get back is the owner of the procedure. 

>: create or replace procedure user1.procedure2 as
   schema_name  varchar2( 30 ) := sys_context( 'USERENV', 'CURRENT_SCHEMA' ) ;
begin
    dbms_output.put_line( 'UserEnv.CurrentSchema = (' || schema_name || ')' );
    . . .
end;
/

Procedure created.

>: alter session set current_schema = user2 ;

Session altered

>: select sys_context( 'userenv', 'current_schema' ) schema from dual ;

SCHEMA
------
USER2

>: exec user1.procedure2 ;
UserEnv.CurrentSchema = (USER1)
. . .

No matter what I set my current schema to, the procedure always sees the "current" schema as the owner of the procedure.