How to check a PDF checkbox from Access vba?

I am filling up PDF forms from Vba in Microsoft Access. Everything goes fine, except that I am unable to check checkboxes from my code. My code is pretty simple, something like that :

Set field = jso.getField("checkboxfieldname") field.Value = True

I tried everything I could think of: 1, "1", "On, "on", True, "checked", etc. Nothing seems to work. I looked at the SDK: nothing on how to check checkboxes from vba in the documentation, nothing in examples either...

I'm sure the answer is simple :-) Thanks for your help, Emphyrio