page items as part of select for classic report

I have a classic report using the following sql

SELECT SUBSCRIBER_NAME,

       CONTENTID

FROM VWINF_CONTENTSUBSCRIBERS

WHERE  contentid = :P15_XCONTENTID

UNION

SELECT "SUBSCRIBER_NAME",

       3<----------------------

FROM WINF_SUBSCRIBERS

WHERE SUBSCRIBER_NAME NOT IN (

SELECT SUBSCRIBER_NAME

FROM VWINF_CONTENTSUBSCRIBERS

WHERE CONTENTID = :P15_XCONTENTID AND SUBSCRIBED = 1)

This runs fine.

I want to replace 3 with ::P15_XCONTENTID it then fails

Is what I am doing 'Allowed'