A camshaft phaser includes a housing with a harmonic gear drive unit disposed therein. The harmonic gear drive unit includes a circular spline and a dynamic spline, a flexspline disposed radially within the circular spline and the dynamic spline, a wave generator disposed radially within the flexspline, and a rotational actuator connectable to the wave generator. One of the circular spline and the dynamic spline is fixed to the housing. A hub is rotatably disposed radially within the housing and attachable to the camshaft and fixed to the other of the circular spline and the dynamic spline. An anti-rotation means is provided for temporarily fixing the circular spline to the dynamic spline in order to prevent relative rotation therebetween when the hub is being attached to the camshaft. The anti-rotation means prevents damage to the harmonic gear drive unit while the camshaft phaser is being installed onto the camshaft.