BUG #11661: CASE ELSE is evaluated although condition is true

The following bug has been logged on the website:

Bug reference: 11661
Logged by: Michael Stieler
Email address: michael(at)aplaypowered(dot)com
PostgreSQL version: 9.2.6
Operating system: RHEL
Description:

I am not sure if it is a bug, a lack in documentation or just a
misunderstanding.
This issue refers to a StackOverflow question at
https://stackoverflow.com/questions/26297035/posgtres-case-condition-with-sum-aggregation-evaluates-not-needed-else-part

The following query:
case when sum( column1 ) = 0
then 0
else round( sum( price
* hours
/ column1 ), 2 )
raises a division by zero error, although the condition sum(column1) = 0 is
true. I would expect that the else part would never be evaluated.
Documentation only describes that this happens for immediates.