OAM Resource Authentication Policy Failure URL to APEX?

I am trying to use Oracle APEX as the "failure URL" for my Authentication Policy in OAM, so that when there is a failure during login, it will redirect the user to a custom APEX error page.

I am currently setting the Failure URL to:   /site/f?p=100:999::::::

so when an error occurs, the user gets redirected to a real URL like:   /site/f?p=100:999::::::&p_error_codes_list=OAM-5,OAM-5&p_error_code=OAM-5

When testing a failure, I do see it redirect to the APEX page in the browser (Firefox Developer Network tab), but every time it directs to the APEX page, it then tries to re-authenticate to OAM, and therefore goes into an endless loop.