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