In non-relativistic QM, the probability postulate works very well in position and momentum space. But, I have read that in relativistic QM (Dirac or QFT), the probability postulate in position space does not work because the corresponding probability density function is not Lorentz covariant. But, what about probability in momentum space in relativistic QM? Is a probability interpretation also not possible there - is it also not Lorentz covariant? Thanks in advance.