I get the same message too. This is what you need to do...

Select how you want to pay emirates Amex or other card... Then select the number of miles you want to purchase you will see NaN... Just press the continue button. It will take you to another page, I'm not viewing screen at the moment, select the currency you want to pay in, it will then calculate a figure

Now add all your card details and go pay... That's it and this should work as I have bought miles from Emirates twice before