In this paper a method for designing robust offset-free MPC controllers for (possibly) nonzero targets is presented. The proposed controller is guaranteed to track the controlled variable to its target for any plant that lies in a polytopic region. First, an off-line design of a robust unconstrained offset-free controller is accomplished, and a corresponding invariant region is computed in which this controller is well defined and does not violate the constraints. Next, the online implementation requires the use of this unconstrained controller if the system state is in the invariant region or, if not, the solution of a min-max finite horizon optimization problem. An illustrative example is presented.