We consider a two-user multiple-input single-output interference channel in which the receivers treat interference as additive noise. The transmitters are assumed to have imperfect channel state information to the receivers. The transmitters choose their beamforming vectors considering worst case power gains at the receivers. We provide a real-valued parametrization of the beamforming vectors that achieve the Pareto boundary of the rate region with channel uncertainties. Simulation results and high SNR analysis show that the gain in spectrum sharing under imperfect channel state information converges to the setting of time division multiple access. Moreover, we provide analytical results for the maximum sum rate in asymptotic cases of low and high signal to noise ratios.