Trying the new MIDIEventList.Builder. Just instantiating it makes it crash. I've tried different word sizes too. let b = MIDIEventList.Builder(inProtocol: ._2_0, wordSize: 32) Here's the error and stack. Can't do much with it unless you can get an instance. I suppose the answer is "wait for beta 54" // Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0) //        (lldb) bt //        * thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0) ////            frame #0: 0x000000010ea55544 libswiftCoreMIDI.dylib`(extension in CoreMIDI):C.MIDIEventList.Builder.allocating_init(inProtocol: C.MIDIProtocolID, wordSize: Swift.Int) -> (extension in CoreMIDI):C.MIDIEventList.Builder + 148 Some good news: The Packet builder does work. let pb = MIDIEventPacket.Builder(maximumNumberMIDIWords: 3)