Postgresql resorting to scans for single term full text searches, not sure why...

gistfile1.txt

alter function to_tsvector(regconfig,text) cost 1000;
set enable_indexscan=false;

explain select * from proxyrec where to_tsvector('english', cs_ua) @@ to_tsquery('mozilla') order by ts limit 1
QUERY PLAN
----------
 Limit (cost=347040.40..347040.40 rows=1 width=1052)
   -> Sort (cost=347040.40..347178.66 rows=55302 width=1052)
   Sort Key: ts
   -> Bitmap Heap Scan on proxyrec (cost=603.09..346763.89 rows=55302 width=1052)
        Recheck Cond: (to_tsvector('english'::regconfig, (cs_ua)::text) @@ to_tsquery('mozilla & perl'::text))
        -> Bitmap Index Scan on proxyrec_text_ua (cost=0.00..589.27 rows=55302 width=0)
             Index Cond: (to_tsvector('english'::regconfig, (cs_ua)::text) @@ to_tsquery('mozilla & perl'::text))

set enable_indexscan=true;

explain select * from proxyrec where to_tsvector('english', cs_ua) @@ to_tsquery('mozilla') order by ts limit 1
QUERY PLAN
----------
 Limit (cost=0.00..11511.21 rows=1 width=1051)
   -> Index Scan using proxrec_tssip on proxyrec (cost=0.00..636592669.35 rows=55302 width=1052)
        Filter: (to_tsvector('english'::regconfig, (cs_ua)::text) @@ to_tsquery('mozilla'::text)