CASE in where statement. BUG ??

I belive that I have found a bug - or maybe it was done on purpose. Have
a look at this query:

SELECT integer_field FROM bugtable WHERE (CASE WHEN '' <> '' THEN
integer_field = '' ELSE integer_field = 0 END);
Result: ERROR: invalid input syntax for integer: ""

SELECT text_field FROM bugtable WHERE (CASE WHEN '' <> '' THEN
text_field = '' ELSE text_field = '1' END);
Result: 1 rows fetched (0,02 sec)

Where table structure is:

CREATE TABLE "public"."bugtable" (
"integer_field" INTEGER,
"text_field" TEXT
) WITH OIDS;

I belive that PG reaction should be the same as in the second query --
there shouldn't be any errors.

$ psql --version
psql (PostgreSQL) 7.4
contains support for command-line editing
Windows XP/Cygwin

If this is not a bug, please tell me what is wrong.

ML