Opportunity Validation Rule on Primary Campaign

Question

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)

Answer 1

Instead of using the ISNULL function, you should use the ISBLANK() function. It is better to use it for text fields (lookup is considered as a text field).

Reference: https://help.salesforce.com/apex/HTViewHelpDoc?id=customize_functions_i_z.htm&language=en_US#ISBLANK