sys_context in fine grained audit not working

I am trying to set up a FGA on a number of tables and want to limit the number of selects to audit by excluding internal selects in pl/sql packages and for that I am using SYS_CONTEXT feature. But, I can't get this to work at all and to make sure that it really doesn't work I did the simplest and most stupid example and it looks like this:

-- Create test table

create table MY_SCHEMA.TABLE_SYS_CONTEXT_TEST

(column1 varchar2(10),

column2 varchar2(10),

column3 varchar2(10),

column4 varchar2(10));

-- add FGA to be triggered where sql statement is null and that should be impossible