Invalid GUI Input data for protected field

I have a input field in VA01/02 defined as:

inputfield (10,3) "PO Number" (10,25) name="z_va01_ ponum" size="25" -
required -uppercase maxlength="35"

When the field is populated with up to 25 characters, everything works fine. However, if I enter more than 25 characters and hit enter (or perform any other action, like SAVE), GuiXT throws me out of the Order, with a "Invalid GUI Input data:" error. I've tried this in my test transaction that is essentially a blank SAP screen, and I get the
same result.

Any ideas on how to fix it? Space is limited, so I'd rather not make it a fixed 35 space field if I can help it.

Thanks,