invalid length parameter passed to the left or substring function

 CASE
               WHEN PATINDEX('/courses/%/pages/%', url) >= 1 AND PATINDEX('%?%', url) >= 1
                    THEN SUBSTRING(LEFT(url,CHARINDEX('?',url)), CHARINDEX('/pages/',url,0)+7,(CHARINDEX('/', REVERSE(url), 1)-CHARINDEX('?', REVERSE(url), 1))-1)
               WHEN PATINDEX('/courses/%/pages/%', url) >= 1 AND PATINDEX('%?%', url) = 0
                    THEN SUBSTRING(url, CHARINDEX('/pages/',url,0)+7,1000)
               WHEN PATINDEX('/courses/%/pages', url) >= 1
                    THEN 'pages-list'
               ELSE ''
          END AS wiki_url