Computed Columns In Temp Tables

I am having a problem with using UDF as part of a temp table computed column. Here's the sample code:
IF EXISTS( SELECT 1 FROM information_schema.routines WHERE routine_name = 'fn_test')
DROP FUNCTION dbo.fn_testGO
CREATE FUNCTION dbo.fn_test( @x int, @y int)
RETURNS INT AS
BEGIN
DECLARE @z INT
SET @z = @x + @y
RETURN @z
END
GO

CREATE TABLE #X
(
x INT,
y INT,
z AS (dbo.fn_test(x,y))
)
I receive the following error:

Server: Msg 208, Level 16, State 1, Line 2
Invalid object name 'dbo.fn_test'.

I do not get this error if I use a regular table.
HELP!