The dipole of the cosmic microwave background (CMB) defines a reference frame for cosmology. Is is assumed since its discovery that the CMB dipole is caused by the proper motion of the Solar system. This hypothesis leads to the prediction that the corresponding Doppler shifts and abberation effects are universal to all frequencies. Thus the CMB frame is assumed to be the comoving frame of freely falling Friedmann observers, which is essential in the analysis of many cosmological observables such as the Hubble diagram. As any fundamental hypothesis, also the proper motion hypothesis must be tested. I present results from a suite of cosmic radio dipole measurements based on radio continuum catalogues across frequencies. We find that the cosmic radio dipole agrees with the direction of the CMB dipole within errors but has an excess in amplitude which increases with wavelength. The limitations and consequences of our finding are discussed.