I only half expect to ever get here. I've never seen a report on the ML where it was due non-invertibility, but not many people do a pure MA model or a model with k_ma > k_ar. We compute the MA start_params using the Hannan-Rissanen procedure from the references (at least my interpretation of it). The problem is that if we call inv_transparams on the ARMA parameters and they're either non-stationary or non-invertible, this introduces NaNs and that's how we end up with the non-convergent SVD error.