Webform missing mandatory fields do not alert

Hi there,
we are using Drupal 5.1 with the Webforms module. Everything is working great except for one thing: When fields on a webform are marked as mandatory and they are not filled out, there is no error message or any other kind of alert to let the user know that the form has not been submitted; it simply refreshes the form. This leads a lot of people to believe that they successfully submitted the form.

I have not been able to find any code that handles alerting in the module. Is this a setting I have overlooked or is this something I have to implement myself? Can anybody shed some light on this for me?

Thanks a lot!