Primary Key Cannot Take NuLL Value

hey guys. i have a field by the name of ISSUE. i am using this afterupdate event procedure
which is as follow:

Private Sub ISSUE_AfterUpdate()
If Not IsNull(ISSUE) Then
ISSUE = Format(ISSUE, "000000")
End If
End Sub

if a user enter like (23) in my ISSUE field my procedure will run and it will replace
the number by (000023) to make it a six digit. i wanted to create a primary key so then a user cannot enter the same number again
or cannot duplicate a record but i cannot because primary key cannot take a null value. Can you all tell me anything to add in my procedure so that once the number is entered like (23)
another user cannot enter (23) again in the record.
Thanks