Issue with creating a data entry form using dmax - doesn't pick up value

Hi all,

I'm creating a simple data entry for that is used to add new data to a pre-created table

I create the form and set the property to data entry

I've added the code for DMAX/I've tried DCount too

=DMAX("PupilID","tblPupil")+1

this finds the largest pupil number and adds one

I've done this before and never had the issue but it is giving an error message (see below) 

I have 13 records - the dmax finds the largest and then adds one on

it automatically shows 14 but as soon as I add another field and try to go to design view or save it thinks there is no value in the ownerID field

I've tried different number formats, key on, key off, delete relationships etc but no joy - any help greatly appreciated