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?