Query not using index for user defined type

Hi guys.
I created a type 'mytype' (an unsigned int) and created an operator class
for index.
Then I created a table with a column of my type and isnerted 1000 entries.
But no matter how many entries I have in the table, it never uses the
index. It always does a seq scan.

Here is the explain analyze with 1000 entries:

explain analyze select * from mytable where a > 120::mytype and a <
530::mytype;

---------------------------------------------------------------------------------------------------
Seq Scan on mytable (cost=0.00..19.02 rows=400 width=4) (actual
time=0.023..0.229 rows=409 loops=1)
Filter: ((a > '120'::mytype) AND (a < '530'::mytpe))
Total runtime: 0.297 ms

Does anybody know why?