Q: using generate_series to fill in the blanks

I've got a desired output which looks something like this..

vdt | count
------------+-------
1 | 514
2 | 27
3 | 15
4 | <NULL>
5 | 12
6 | 15

the query in psql is something like this..

select vdt, count(*) from footable where c_id = '71' group by vdt order
by vdt

problem is.. since there's not data whatsoever on vdt=4 I get this..

vdt | count
------------+-------
1 | 514
2 | 27
3 | 15
5 | 12
6 | 15

I tried to use generate_series

select generate_series(1,7,1), count(*) from footable where c_id = '71'
group by generate_series(1,7,1),vdt order by generate_series(1,7,1);

(note : the vdt are numbered from 1 to 7 sequence)

generate_series | count
-----------------+-------
1 | 514
1 | 27
1 | 15
1 | 12
1 | 15
2 | 514
2 | 27
2 | 15
2 | 12
2 | 15
3 | 514
3 | 27
3 | 15
3 | 12
3 | 15
4 | 514
4 | 27
4 | 15
4 | 12
4 | 15
.....
[snip]
.....