An electric resonant circuit is provided in a receiving part of a coupling for a fluid line, the resonant circuit interacting with a tuning device in a contactless manner. In an intermediate position of an insert part, which differs from a closed position, the resonant circuit has a resonant frequency that differs from the characteristic closure frequency, and in the closed position the resonant circuit has the characteristic closure frequency. The tuning device acts on the inductance of the electric resonant circuit. An acting part is provided that mechanically acts on the coil when the insert part is inserted into the receiving part such that the shape of the coil in the closed position differs from the shape of the coil in the intermediate position.