Making a new Item Type Mandatory

Hello All, So I have created 2 Lists (project_id & SystemLevel) and filled them with appropriate values. I have then added these to the list of properties in the Part Item Type. I then linked these to combo boxes and am happy with how they appear on the form. These two Lists are used to create a portion of a part number which they do. The problem is that I would like to make these combo boxes mandatory. When I select the required box and save, I get the following: Cannot insert the value NULL into column 'PROJECT_ID' etc. I'm guessing that the default value is blank when the Part form is loaded, which is not allowed. What I'm trying to achieve to ensure that this field is selected at the point of save, therefore its okay of field is blank when the form loads. How do you get round this? Cheers