UNION ALL query help

I am received the error message:

Unknown column 'title' in 'field list'

from the query below.  I am not understanding what I have done wrong.  The
goal of the query is with for a RSS feed determining the 15 most current
articles.

Ron



SELECT title, content_date, content, reference FROM ( (

SELECT `page_title`, `last_update`, 'article', `reference` FROM
`user_pages` WHERE `include_in_highlights` =1 ORDER BY `last_update` DESC
LIMIT 15

) UNION ALL (

SELECT `blog`.`entry_title`, `blog`.`entry_date`, `blog`.`entry`,
`blog`.`reference` FROM blog WHERE blog_owners_reference =1 ORDER BY
blog.entry_date DESC, blog.reference DESC LIMIT 15

) ) AS rss_feed ORDER BY rss_feed.content_date DESC LIMIT 15