How to trigger message box with dropdown selection?

I am trying to trigger a message box when I select "No" as a dropdown selection using the change event:

if(this.rawValue = "No"){xfa.host.messageBox("Please try again.","Error")}

However, it is triggered even when I select "Yes." Perhaps it may be due to the fact that the following script is in the "exit" event:

if(this.rawValue==null){this.execValidate();}