nth_value is incorrect for some initial values

Nth_value is returning null when not expected. For example:

select e1, nth_value(e1, 2) over (order by e1 nulls last) from g1

with our default data set should return 'a' for each nth_value, but instead returns null for one of the 'a' rows. The issue is that we should be considering the indexing to be relative from the start of the peer group in the frame rather than from the current index of the row.