The FIR (Far-Infrared) polarimeter at JET is affected by an anomaly which makes difficult the interpretation of both Faraday and Cotton-Mouton effect measurements. The anomaly is clearly displayed during the calibration operations in absence of plasma: as the polarization of the probing beam is rotated, the phase shift of the polarimetric signal with respect to the interferometric signal is not constant, as expected, but changes significantly. It affects all the polarimetric measurement channels and has been so far removed by an empirical preprocessing of the raw data. It can be ascribed to a non ideal behaviour of some optical components. Looking for a possible explanation of the anomaly, in this paper we analyse the optical set-up of the JET interferometer-polarimeter according to the laws of the classical polarization optics. At first, the optical characteristics of the recombination plates are analysed in detail. Although they produce ellipticity in the transmitted and reflected beams, the results show that the recombination plates should not be responsible of the anomaly of the polarimeter. Then, the dielectric waveguides used to transfer the recombined beams from the Torus Hall to the detectors are, for the first time, considered as a possible origin of the anomaly. The anomalous behaviour is expected to be mainly originated by reflections on metal mirrors which may produce rotations of the polarization of the beams. A calculation has been performed in order to analyse the effects of a rotation of the polarization of the recombined beam on the detector signals. As a result, a rotation of the polarization along the line could explain the anomaly. We also suggest some simple and feasible tests, useful to give an experimental support to this conclusion, and discuss possible modifications of the optical set-up to remove or greatly reduce the anomaly in future measurements.