APEX

Hi,
I am using Application Express 4.0.0. I am struggling with a strange problem while trying to implement custom authentication schema.
I declare a page sentry function 'page_sentry' which returns TRUE or FALSE based on certain conditions. My page_sentry is as follows:

FUNCTION PAGE_SENTRY RETURN BOOLEAN
IS
l_username VARCHAR2(512);
l_session_id NUMBER;
BEGIN
IF USER != 'APEX_PUBLIC_USER' THEN
RETURN false;
END IF;
l_session_id := wwv_flow_custom_auth_std.get_session_id_from_cookie;
-- check application session cookie.
IF wwv_flow_custom_auth_std.is_session_valid THEN
apex_application.g_instance := l_session_id;
l_username := wwv_flow_custom_auth_std.get_username;
wwv_flow_custom_auth.define_user_session(
p_user => l_username, p_session_id => l_session_id);
RETURN true;
ELSE
--redirect to login page using OWA_UTIL.REDIRECT_URL
END IF;
RETURN false;
END page_sentry;

And Cookie Name : _AUTH
At first, It always returned FALSE. I wasn't getting any error. However, even after forcibly returning TRUE from the page_sentry function the redirect was still not happening. I tried to look into what cookies were being set for the same and I found this:

Name: ApexLibErrorStack1
Content: page%3D1%3Cbr%20%2F%3EERR-1201%20session%20ID%20not%20set%20on%20custom%20authentication

I couldn't find any relevant help for this. Worst of it is I cannot set any cookie from the page_sentry function at all. Please help!