The invention relates to a conductor assembly comprising a resilient tubular outer casing (1) with a flexible printed circuit board (6) in the outer casing's interior. On the circuit board a sensor (3) is arranged. A cavity (8a-c) in the outer casing adjacent to the sensor is filled with a rigid filling material (9). Where the rigid filling material extends in the lengthwise direction of the tubular outer casing (1) at least from one side of the sensor to the opposite side of the sensor. This means that the sensor will not be affected when the conductor assembly is bent or otherwise affected mechanically. The conductor assembly's outer casing (1) comprises an opening (7a-c) to a cavity (8a-c) in the outer casing in connection to the sensor, through which a filling material can be supplied in liquid form. Typically the conductor assembly includes two cavities (8a-b) in the outer casing on the side of the flexible circuit board (6) where the sensor is mounted, the two cavities being on either side of the sensor, and a third cavity (8c) in the outer casing on the side of the flexible circuit board (6) which is opposite to the sensor. In an advantageous embodiment the extension of at least one cavity (8a-c) in the outer casing is limited by bumps (4a-f) on either side of the cavity thus defining the cavity. Injected filler material propagation is governed by these bumps to the relevant space. Typically, the conductor assembly is a catheter or part of a catheter.