gql error: BadQueryError: Parse Error: Invalid ORDER BY Property at symbol :3

I'm getting this error:

Parse Error: Invalid ORDER BY Property at symbol :3

Can't see what is causing it, if I remove the substitution and add "date" it works.

order = "date"
playlists开发者_运维百科_data = db.GqlQuery('SELECT * FROM Playlist WHERE 
                              client = :1 AND 
                              category = :2 
                              ORDER BY :3 DESC', client, category, order)

ORDER BY doesn't support bound parameters.

They can only be used in a WHERE clause, and only to replace property values, not property names.