Unexpected behavior with CASE statement

Suppose I have the following table named "metrics":

metric_type | val
------------+-----
0 | 1
0 | 1
1 | 0
1 | 3

Now suppose I run the following simple query:

select
metric_type,
case metric_type
when 0 then
sum (1 / val)
when 1 then
sum (val)
end as result
from metrics
group by metric_type

I expect to get the following result set:

metric_type | result
------------+-------
0 | 2
1 | 3

But in reality I get the following error:

ERROR: division by zero
SQL state: 22012

So it appears that Postgres executes all cases and select the result
in the end. Is this expected behavior?

Thanks
- Jimmy