The invention relates to a urea supply system for a waste gas cleaning catalyst of an internal combustion engine, comprising a urea tank for the receiving of urea solution, a connecting line that connects the urea tank with a waste gas cleaning catalyst, a pump in order to pump urea solution via the connecting line from the urea tank to the catalyst, and a heating insert for the defrosting of frozen urea solution. According to the invention it is provided that the return line is configured and arranged in such a manner that, during operation, because of the urea solution discharged from the return line there is prevented the forming of an air gap between the heating insert and the frozen urea solution. The invention relates also to a heating insert for such a urea supply system.