How to disable fields on Business Process Flow

Hi All,

I want to disable some fields on Business Process Flow. I tried to use the following code in form onload.

Xrm.Page.getControl("header_process_tk_typeofoffer").setDisabled(true);

The above code is giving the following error

I can see one point here

• If I apply the above code for the field which exist in First stage then it is working
• If I apply the above code for the field which exist in other than First stage then it is not working

here my question is can't we disable a BPF fields that exist in other than first stage?

Thanks,

Phani.