Conditional Fields on Purchase Order Lines does not work

Conditional Fields on Purchase Order Lines does not work when trying to make Part description read only based on 'Inventory Part' flag status. (APP9)

1. Go to purchase order window
2. RMB lines > Conditional Fields
3. Select 'Part Description' from the left selection pane
4. Click on 'Add property action' on right side
5. Change property to ' Ready only'
6. Add criterion where Type:1, column:'Inventory part', operator:'=', value:True
7. Click ok and reload window.
8. Add new PO line for an inventory part (where Inventory part is checked) and modify the part description