A method for sealing an electric coupling piece (K) against moisture, includes, at an end of the line (1) intended for connection to the coupling piece (K), at least one annular groove extending in the circumferential direction is cut into the casing of the line (1). Prior to injection molding the protective body (9), an O-ring (10) serving as a sealing element is placed into the groove, where the O-ring (10) rests tightly against the lateral borders of the groove (11) with the respective pretension, where the O-ring is of a material which connects tightly and in a moisture proof manner to the insulating material of the finely produced protective body (9).