When this happens, especially in a linked Customer record, it is caused by the Customer having an invalid Contact or ShipTo linked that does not actually exist.  

I wrote a BAQ that will show this to a user that is having the issue.  BAQ ID = GlobCustIssue_InvalidLinks 

To fix the issue:

• Open the relevant Customer record
• Set one of the contacts in the list as the primary contacts, over-riding any that are set but do not exist on the Customer record.  If you check all 3, save, then uncheck them, and save again, I believe that will also fix the issue, but it is best to set those contacts correctly for the Customer if you can.