SQL & PL/SQL

I am trying to apply a policy predicate to a table to restrict the columns users see when executing a select query. I have created the context and can select from it using DUAL in a SQL session, the package to create the predicate is as follows

CREATE OR REPLACE PACKAGE BODY XYZ
AS

function ABC(D1 varchar2,D2 Varchar2)
Return Varchar2 IS

d_predicate varchar2(2000);
v_value varchar2(1) := 'Y';

BEGIN
d_predicate := 'SYS_CONTEXT("MY_CONTEXT","MY_VALUE") = v_value';

RETURN d_predicate;

END ABC;
END XYZ;

Each time I select from the table I get the error above, can anyone see the problem?

Thanks in advance
Tina