case not sensitive to null condition ?

v9.0.1 on linux

thedb=# select
''''||bicolumn||'''',
coalesce(permitted_values,'is_null'),
case permitted_values when NULL then 'null' else ''''||permitted_values||'''' end
from bi_constraints limit 2;

?column? | coalesce | case
----------------+---------------+-----------------
'block' | is_null | <--- should be 'null' ??
'design_style' | rls,analog,fc | 'rls,analog,fc'
(2 rows)

Why didn't the case return 'null' in the statement above?

Thanks in Advance !