Phase margin with several gain crossover frequencies

Dear all, when in a system, we have several gain crossover frequencies, naturally we'll have several phase margins. But margin command in matlab does not always compute the minimum one or the one corresponding to the minimum gain crossover frequency. I think this is a big bug in matlab 2013a. How I can compute the phase margin corresponding to the minimum gain crossover frequency in FRACTIONAL-ORDER systems?

Thanks a lot,