When adding a comment to a talk but do not specify a rating, your get an message stating that you did not select a rating. The message-field shown directly below the error is empty.
At that point, the user will either re-enter the comment (unuserfriendly), press the back button and hope the browser still has the text in the original form (unreliable), or give up and walk away (undesired).
add client-side validation prior to submitting the form
show the submitted data in the form field on the page showing the error