Predicted value of RM as a function of the emission radius for a RIAF toy-model fitted to the ALMA data. The background RM corresponding to the value observed toward the magnetar J1745−2900 is assumed. Accounting for the relativistic correction f_e decreases the RM significantly, by a factor of 20 for r_e = 4 r_g, but does not preclude the dominant contribution to the Faraday rotation from the innermost part of the flow.