fix crash on arm64 devices

Function Pointers Must Use the Correct Prototype:
https://developer.apple.com/library/ios/documentation/General/Conceptual
/CocoaTouch64BitGuide/ConvertingYourAppto64-Bit/ConvertingYourAppto64-Bi
t.html#//apple_ref/doc/uid/TP40013501-CH3-SW22