locate and replace problem

Hello,

I need help. In my trigger is not function LOCATE and REPLACE.Locate always
return 0. If I change :OLD.Name and NEW.Name to string like for example
'Mike', it's run.

Thanx David

CREATE TABLE admin.sipuser
(
    Id               Fixed (11,0)           NOT NULL  DEFAULT SERIAL (1),
    Name             Varchar (80)  ASCII    NOT NULL  UNIQUE,

    PRIMARY KEY (Id)
)

CREATE TABLE admin.extension
(
    Id           Fixed (11,0)          NOT NULL    DEFAULT SERIAL (1),
       SipUser_id   Fixed (11,0),
    AppData      Varchar (80) ASCII    NOT NULL    DEFAULT ' ',
    PRIMARY KEY (Id)
)
//
ALTER TABLE admin.extension
    FOREIGN KEY fk_sipuser_id (SipUser_id) REFERENCES admin.sipuser (Id) ON
DELETE CASCADE

CREATE TRIGGER sipuser_update FOR sipuser AFTER UPDATE EXECUTE
(
 UPDATE admin.extension
   SET AppData = REPLACE (AppData, :OLD.Name, :NEW.Name)
   WHERE SipUser_id = :OLD.Id AND LOCATE (:OLD.Name, AppData) <> 0;
)