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 !