The definition of the helicity of a massless relativistic particle as the projection of the spin onto the momentum, ${\chi}^{mom} ={\bf s}\cdot\hat{\bf p}$, is equivalent, in the free case, to defining it as ${\chi}^{vel} ={\bf s}\cdot{\bf v}$, where ${\bf v}$ is the velocity. However when the effective mass induced by the spin-field coupling does not vanish, neither of these expressions is conserved in general except in a pure magnetic field, as is illustrated by the motion of a massless particle with gyromagnetic ratio $g=2$ in a constant pure electric field. Both helicities vary during an initial period, but for $t\to\infty$ both expressions converge to the same constant value.