In which field you have done your graduation i think in same field you should do your post graduation because may be you may face problems in your future so before you take any decision think twice.
If you want to choose MBA as your passion then make yourself capable to prove that many of the people do choose different fields and some of them achieve and some may fail.
Nothing wrong as long as you very clear as to why you want to do it and what career path you want to persue. Check your real passion and move on. Choose your specialization that gels with your career ambitions.
All the best,