microsoft access training class - Primary key

I have recently put a new client into the database. All ok.
Then I need to create a new form, with client details of monthly sales. As soon as I go to save I am told that the "Index or Primary key cannot contain a null value."
However I have put one in the database. It is a reference code made up of the clients name.
We have been using this system for quite awhile. It was originally set up by someone who has since left the company and the Primary key has been established with using the clients name as the code.
What am I doing wrong?
Any advice will greatfully received.