Cannot create a record in . The record already exists.

I have created a custom table, it contains 2 fields: A custom code (DS72), which uses a number sequence I have setup and ItemId. I have joined this table as a datasource to the InventTable form and am displaying the value held in this for any associated Item's in the grid view. The problem is, when I edit an item which doesn't have a DS72 code attached, I get the following error:

Cannot create a record in BTN_DS72 (BTN_DS72). DS72: .

The record already exists.

When I go to the DS72 form, it seem's to have created or tried to create a new entry for some reason? I don't know why it does this?