nth_value out of more than n values returns null

[Guyren Howe]

This query:

SELECT NTH_VALUE(id, 5000000) OVER (ORDER BY created_at, id ASC) FROM table

in a table where SELECT COUNT(*) returns a value a few thousand over 5 million, 
where id is the primary key, returns null.

The inclusion of the primary key should make the order by a total order. So 
there should be a 5 millionth row.

How can this happen?