Invalid answer drag and drop exercise

Hello ! 

I am developing a Drag and Drop Exercise with one correct answer. When I click on the submit button without completing the drag and drop I have the pop-up "Invalid answer" with the text: "You must complete the question before submitting". And this is what I want, but if I move a little one of the answer without dropping it in the drop target, the Drag and Drop interaction is submitted when I click on the submit button even if I dropped none of the answers in the dropped target. How can I make the "Invalid answer" pop-up appears when the user moved an answer without dropping it in the dropped target?

Thanks a lot!