On the Stronger Statement of Levinson's Theorem for the Dirac Equation

Recently a stronger statement of Levinson's theorem for the Dirac equation was presented, where the limits of the phase shifts at $E=\pm M$ are related to the numbers of nodes of radial functions at the same energies, respectively. However, in this letter we show that this statement has to be modified because the limits of the phase shifts may be negative for the Dirac equation.