pg_get_triggerdef can't find the trigger using OID.

testdb=# SELECT 

testdb-#   p.oid,

testdb-#   n.nspname as "Schema",

testdb-#   p.proname as "Name",

testdb-#   pg_catalog.pg_get_function_result(p.oid) as "Result data type",

testdb-#   pg_catalog.pg_get_function_arguments(p.oid) as "Argument data types",

testdb-#  CASE

testdb-#   WHEN p.proisagg THEN 'agg'

testdb-#   WHEN p.proiswindow THEN 'window'

testdb-#   WHEN p.prorettype = 'pg_catalog.trigger'::pg_catalog.regtype THEN 'trigger'

testdb-#   ELSE 'normal'

testdb-# END as "Type"

testdb-# FROM pg_catalog.pg_proc p

testdb-#      LEFT JOIN pg_catalog.pg_namespace n ON n.oid = p.pronamespace

testdb-# WHERE pg_catalog.pg_function_is_visible(p.oid)

testdb-#       AND n.nspname <> 'pg_catalog'

testdb-#       AND n.nspname <> 'information_schema'

testdb-# ORDER BY 1, 2, 4;

  oid  | Schema |  Name   | Result data type |                     Argument data types                      |  Type   

-------+--------+---------+------------------+--------------------------------------------------------------+---------

 18249 | public | test_f  | trigger          |                                                              | trigger

testdb=# select pg_get_triggerdef(18249);

ERROR:  could not find tuple for trigger 18249