For some reason or another (money) Apple makes it so that all their computers don't accept normal types of ram. This could be for compatibility reason, or for standard performance, most mostly for money.
SO with this in mind be careful when buying ram for your Mac. And beware of listings for Mac ram by users instead of companies, users can get their information wrong, and in that case, waste your money.
I would recommend
were you can find the right type of ram for your exact model