Calculate a quotient for a count of boolean values (true or false)

Hello!

In a PostgreSQL 8.4.13 why doesn't this please
deliver a floating value (a quotient between 0 and 1):

select
id,
count(nullif(nice, false)) - count(nullif(nice, true)) /
count(nice) as rating
from pref_rep where nice is not null
group by id
;
id | rating
-------------------------+--------
DE10072 | -1
DE10086 | 18
DE10087 | 1
DE10088 | -1
DE10095 | 276
DE10097 | 37
DE10105 | 5

I am trying to calculate a sum of ratings
given by users to each other
to draw a pie chart in a game, more details here:

http://stackoverflow.com/questions/17040692/calculate-a-quotient-in-one-table-and-store-it-in-another-table

Thank you