format() function with string_agg

I'm trying to use format() function with string_agg with Postgres 9.1.9 but
getting error - "too few arguments for format"

I have two tables t_message and t_message_args. Table t_message has
'message' column with text in the format 'Test message first arg is %1$s
and second arg %2$s end-of-message'

id | integer |
message | character varying |

And the second table t_message_args contains message argument and values
id | integer |
arg | integer |
argvalue | character varying |

Here is the SQL that uses format() function to retrieve formatted message
by replacing arg value in the message:
=
select m.id, format(m.message, string_agg(a.argvalue, ',' order by a.arg)
from t_message m, t_message_args a
where m.id = a.id
group by m.id, m.message

ERROR: too few arguments for format

********** Error **********

ERROR: too few arguments for format
SQL state: 22023