How to call a Stored Procedure which returns a single value?

Hi all,

I would like to create such a select:

SELECT
CASE
WHEN (check_suffix(mt.table_name) = 'T') THEN 'A'
ELSE 'B'
END as app_name,
mt.table_name
FROM
t_user_structures_detail mt;


where check_suffix is a procedure returning VARCHAR.

CREATE PROCEDURE check_suffix (tab_name VARCHAR(10)) RETURNS (result VARCHAR(10)) AS
...

If I use
WHEN (SELECT result FROM check_suffix(mt.table_name) = 'T') THEN 'A'

then I get an error.
Dynamic SQL Error
-SQL error code = -104
-Token unknown - line 4, char 72
-=
Statement failed, SQLCODE = -607


Thanks,
Andrej.