bug or change in functionality in 7.2?

I have in my code a SQL statement that does the following:

select period_start + interval('1 hour') from periods;

This worked in 7.1, but in 7.2 I am getting the following error:

ERROR:  parser: parse error at or near "'"

If I remove the quotes then I get the following error:

ERROR:  parser: parse error at or near "hour"

Was this change from 7.1 to 7.2 intentional?  If so, how should this be
coded in 7.2?

thanks,
--Barry