That is how the Piggy bank works: you first buy the empty piggy bank to fill it up and once it's full you can use the Gold bars (instead of purchasing it once it's full). You can find more info right here and leave your feedback
Also, don't miss out on chances to win extra Gold and Boosters, there's always something to win around here, so keep an eye on our News section! You can find more in the links in my signature below!