Davison's design for the servomechanism is treated anew using the method of the state-space in the frequency domain, mainly developed by Rosenbrock. It is shown that the compensator is not robust with respect to all its parameters. However, if the perturbation of the sensitive parameters is ‘ small’, the error is also ‘ small ’ if the signal to be tracked and the disturbances are bounded. In order to make the reasoning more systematic, the notion of ‘ blocking zero ’ is introduced.