Regarding select distinct ...query

Hello All,

I have a select query with select distinct....this query is giving
me error as "unable to identify ordering operator '<' for type
'unknown'"

when i used this query without distinct, it works fine.
What is actually happening?? Can anyone fed some light on
it...???

SELECT prod_chain.depot_sending as depot_sending,
          prod_chain.depot_sending as description,
          '' as part,
          visual_sc.xpos as xpos,
          visual_sc.ypos as ypos,
             visual_sc.vlevel as    depthlevel,
             visual_sc.parent_hpos as    hparent,
             visual_sc.node_hpos as    hnode,
             visual_sc.visible as nodevisible,
             visual_sc.nameshown as nameshown,
          0 as parentindex
     FROM prod_chain LEFT OUTER JOIN visual_sc
    ON ( TRIM (trailing '' from prod_chain.depot_receiving ) =
TRIM( trailing '' from  visual_sc.parent_depot ) ) and
          ( TRIM( trailing '' from  prod_chain.depot_sending ) =
TRIM( trailing '' from  visual_sc.child_depot ) ) WHERE
          ( ( TRIM( trailing '' from  prod_chain.depot_receiving )
= '' ) AND
            ( TRIM( trailing '' from  visual_sc.initiator_depot )
= '' ) AND
               ( TRIM( trailing '' from  visual_sc.initiator_part ) = '' )
AND
               ( TRIM( trailing '' from visual_sc.user_id ) = '' ) AND
            ( TRIM( trailing '' from visual_sc.sc_type ) = 'B'
)AND
                 visual_sc.vlevel = 2  AND
                 visual_sc.parent_hpos = 0
             )

Thanks and regards,
Prachi