How can i disable DFF by using fnd_descr_flex.update_definition in CUSTOM.pll ?

Hi all ,

I have created descriptive flexfield on RECEIPT form header with name RECEIPT INFORMATION. I want to disable conditionally using FND_DESCR_FLEX.UPDATE_DEFINITION as defined in the 'Oracle EBS Developer Guide'. But unable to do so by using following code in CUSTOM.pll.

if (event_name = 'WHEN-NEW-FORM-INSTANCE') then

IF (form_name = 'ARXRWMAI' AND block_name = 'RGW_FOLDER') THEN

fnd_descr_flex.update_definition

(

BLOCK => 'RGW_FOLDER'

,FIELD => 'DESC_FLEX'

,READ_ONLY => 'Y'

,ENABLED => 'N'

);

END IF;

END IF;

Regards,

Habib