Check for a Variable equal to null.

I want to display a message if a variable is equal to nothing (null). In the Lectora I have an action with a condition that says if the variable equal to (Then I left the last field blank for a null), then display the message.This works in the run and preview modes, but when I publish to HTML, the action does not work. Is there something I am doing wrong? Is there a specific value I need to enter into the condition for null?Thanks in advance for your help.