The help says the item number is optional

If you do this in a Form (without the item number) it works

Code:
        Object oMyForm is a Form
            Set Prompt_Object to oMyPromptObj
However if you do it in a dbForm then the result is variable

Code:
        Object oMyForm is a dbForm
            Set Prompt_Object to oMyPromptObj   // no prompt button

        Object oMyForm is a dbForm
            Set Prompt_Object 0 to oMyPromptObj   // works

        Object oMyForm is a dbForm
            Entry_Item MyFile.MyField           // even though nothing defined in DD for Prompt_Object 

            Set Prompt_Object to oMyPromptObj   // also works without the item number