InfoPath Dev

I have a form that is read only on the infopath form depending on the users group. A non admin can create a new document and submit it to my sharepoint 2010 document library. (I'm using Infopath 2010) However, when the admin updates the form to enter a value in the read only field (which is a people picker) The non admin can no longer submit the form once this field is completed. They get an error message that the system cannot find a lookup value for the field. Please let me know if this is not enough information. Thank you very much!!! Fred

 So - the admin updates the form and re-submits it. Then the non admin opens the form and does ... what? Can they still change it? If it is read only at this point for them, why would they submit it? Sorry, just a li'l confused. :)