interval syntax change from 7.1.2 to 7.2.1?

hi - i recently upgraded my server from 7.1.2 to 7.2.1 and now one of my
apps is no longer functioning. this is the query:

SELECT DISTINCT rate, sum(extract(epoch from interval (end_time -
start_time)))/3600) AS duration
FROM log
WHERE invoice_state='8'
GROUP BY rate HAVING rate > 0

where 'end_time' and 'start_time' are columns in table 'log'

the reported message is:

PostgreSQL said: ERROR: parser: parse error at or near "end_time"

any help greatly appreciated.

jinn