usage of pg_get_functiondef() -- SQL state 42809

-- This works.

select
TRG.tgname, TFX.proname, pg_get_functiondef(TFX.oid) as fdef
from
pg_trigger TRG
inner join pg_proc TFX on TFX.oid = TRG.tgfoid
where
TRG.tgisinternal = true

-- This blows up. -- SQL state: 42809 -- ERROR: "array_agg" is an aggregate function

select
TRG.tgname, TFX.proname, pg_get_functiondef(TFX.oid) as fdef
from
pg_trigger TRG
inner join pg_proc TFX on TFX.oid = TRG.tgfoid
where
TRG.tgisinternal = true
and
pg_get_functiondef(TFX.oid) = 'whatever'

Can you help me understand why this blows up ?
I am running "PostgreSQL 9.1.2, compiled by Visual C++ build 1500, 64-bit"

-dvs-