group by does not show error

I am using Postgresql 9.1.0.

I found that following GROUP BY query works in my DB :-

\d t1
Table "public.t1"
Column | Type | Modifiers
--------+---------+-----------
i | integer | not null
nam | text |
Indexes:
"t1_pkey" PRIMARY KEY, btree (i)

select i,nam
from t1
group by i;

i | nam
---+-----
(0 rows)

I think the query should show an error as I did not maintain group by
standard.
Does anyone know why it does not show syntax error?