EVENTDATA() function is giving only null values in sql server

I'm trying to capture logins time with a trigger using EVENTDATA() function. But It's giving Null values only for all types of events. Can anyone please help me on this? This is my Trigger

CREATE TRIGGER trgBlockIP
    ON ALL SERVER
    FOR LOGON
    AS
    BEGIN
       DECLARE @clientip NVARCHAR(15);
       SET @clientip = (
                          SELECT
                             EVENTDATA().value(
                                                 '(/EVENT_INSTANCE/Clienthost)[1]',
                                                 'NVARCHAR(15)'
                                              )
                       );
       IF EXISTS (
                    SELECT ip
                    FROM   master.dbo.BLockedIPAddresses
                    WHERE  ip = @clientip
                 )
       BEGIN
          PRINT 'Your IP Address is blocked';
          ROLLBACK;
       END;
       ELSE
       BEGIN
          DECLARE @Range VARCHAR(15);
          SELECT
             @Range = SUBSTRING(
                                  @clientip,
                                  1,
                                  LEN(@clientip)
                                  - CHARINDEX('.', REVERSE(@clientip))
                               ) + '.*';
          IF EXISTS (
                       SELECT ip
                       FROM   master.dbo.BLockedIPAddresses
                       WHERE  ip = @Range
                    )
          BEGIN
             PRINT 'Your IP Address Range is blocked';
             ROLLBACK;
          END;
       END;
    END;
    GO