A deformation sensor is described which does not require a supplementary sensor to check functionality. This deformation sensor includes an optical transmission medium (nullM1, nullM2), multiple transmission elements (S1, S2), which couple the light of various wavelengths (null1, null2) into the transmission medium (nullM1, nullM2), and multiple reception elements (E1, E2), which selectively couple the transmitted light out of the transmission medium (nullM1, nullM2) according to wavelength. An evaluation unit (AW) detects deviations between the output signals (a1, a2) of the reception elements (E1, E2) and signals a malfunction of the sensor if the deviations exceed a preset measure.