When you imported your contacts, did you select "Perform auto-enter options while importing"?
If not, your imported contacts do not have the necessary serial number in K_ID_Contact.
You'll either need to re-import your records in a new copy of the databae file and this time select that option or you'll need to put this field on a contacts layout, show all records and use Replace Field Contents in the Records menu to give all your contacts a serial number.
Phil, thanks very much! That did the trick!
You're not the first person to report this issue. Other's have had the same problem with the Contact Manager.
Filemaker Inc. should add some documentation labled "how to import contacts into this file" to their starter solutions that warns people about this.