I must be going crazy. On an Opportunity I want the Validation Rule to trigger when no Primary Campaign has been selected. This is a Standard Field on the Opportunity so the rule I set up is
ISNULL(Campaign.Id)
but I can still edit and save Opportunities that don't have any Campaign selected. I can't figure out what I am missing. The Standard Field is as such
Label Name DataType
Primary Campaign Source Campaign Lookup(Campaign)