Wave Energy Converters (WECs) consisting of oscillating bodies can gain significant benefit from a real-time controller that is able to appropriately tune the system operation to the incident wave, thus allowing for a higher energy capture in a wider variety of wave conditions. Some of the proposed controllers, however, are non-causal and need predictions of the excitation force to be implemented in practise. A frequencydomain model is proposed, for the estimation of the effects that the wave- excitation-force prediction error has on the reference velocity that is calculated from the non-causal control law and, ultimately, on the absorbed power. The model can easily be derived exclusively from the predictor and from the non-causal law, with no additional information about the excitation force. Such a frequency-domain model can be valuable for the design of a robust control architecture, where the prediction error has a limited effect on the performance (power absorption). Focus is put on reactive control, but it is shown how the proposed sensitivity model can be generalised to other non-causal strategies, such as model predictive control.