Cannot set Subform Multiline field to read-only using Client Script

Cannot set Subform Multiline field to read-only using Client Script

I am using client script to set different subform fields as read-only 

It works for Single Line Fields, but when I try to set a Multiline field as Read-only it doesn't work

var subform = ZDK.Page.getSubform("Subform_1");

var item_Code_old = subform.getField('Item_Code_Old').setReadOnly(true);

var quantity_old = subform.getField("Quantity_old").setReadOnly(true);

var description_old = subform.getField("Description_old").setReadOnly(true);

I check the API name multiple times, it's correct

Still I cannot set Multiline field to read-only