At 21.04 hours on 9 Dec 1941, U-134 fired two torpedoes at two ships in a two column convoy with course west near the Norwegian coast and hit the second ship of the left column. The Steinbek was hit by one torpedo amidships and sank shortly thereafter. Schendel thought that he attacked an enemy convoy and reported one steamer of 4000 grt sunk, but a radio message three hours later from the BdU notified him that he had attacked a German convoy in error. The convoy consisted of the four German steam merchants Theresia L.-M. Russ, Magdanena Vinnen, Coburg und Steinbek, escorted by the German armed trawlers Polarsonne, Nordriff and Nordwind.

The OKM investigated the case of friendly fire and came to the conclusion that Schendel made the incomprehensible mistake to think that is it possible that an enemy convoy sails with course west so close to the Norwegian coast under German control. They blamed the commander with the sole responsibility for this incident and overworked the orders for U-boats operating in the Norwegian coastal waters. The BdU came to the conclusion that the commander could not be blamed because he was not informed about the sailing times or the positions of the German ships in that area. It was the first patrol of the U-boat and commander in the relative new combat area of the Arctic waters.