Tables :: Two Part Primary Key With One Null Value

I have just inherited a database that currently has no primary key set up. I wanted to add a two part primary key, but have run into a couple problems. The purpose of the database is to keep track of parts that have come back for repair. I wanted to set the key up to be a combination of the call number and the part serial number. I should always have a serial number, but the serial number can be in the table more than once, as it could have needed repaired more than once. I will normally have a call number for each record.

However, the call number could be repeated more than once, if more than one part came in on the call. The combination between call number and serial number should always be unique (each time a part comes in, it should be on a different call). My problem is arising, when a part is just pulled off the shelf of the factory and sent in for an upgrade. In a case like that, there is no call number. However, a serial number will never come in more than once without a call number.

So the combination of call number and serial number should still remain unique. When I try to set this up in the database, it tells me that the primary key cannot contain a null value (referring to the times that a serial number comes in without a call number). I want to get this set up so that the people entering data in the database will not be able to enter the call number, serial number combination more than once. How can I do this if one part of the primary key can be null?