xfa.host.messageBox

I am trying to get a message to appear if someone tries to enter enter data in a drop down list before entering in formation in a previous list.  I have two drop down lists: ServiceType[0] and Action[0]. I have this code set up in the change event:

topmostSubform.Page2.Action[0]::change - (JavaScript, client)

(topmostSubform.Page2.ServiceType[0].rawValue == null)

{

, "Error", 3, 1);

}

When I select something from the Action[0] list, nothing happens when I have not selected anything in the ServiceType[0] list. What am I missing please?