Vexing crash in AVAudioEngine

I have written the following AVAudioEngine code that initializes one of my custom AUAudioUnit subclasses and wires it up to the main mixer node of the engine.

class AudioEngine {

    let engine = AVAudioEngine()

    init() {

        let sess = AVAudioSession.sharedInstance()
        try! sess.setCategory(AVAudioSessionCategoryPlayback)
        try! sess.setActive(true)


        let desc = AudioComponentDescription(componentType: kAudioUnitType_Generator, componentSubType: 0x00000001, componentManufacturer: 0x00000002, componentFlags: 0, componentFlagsMask: 0)

        AUAudioUnit.registerSubclass(MyAudioUnit.self, as: desc, name: "MyAudioUnit", version: 2)


        let format = engine.outputNode.outputFormat(forBus: 0)

        engine.connect(engine.mainMixerNode, to: engine.outputNode, format: format)

        AVAudioUnit.instantiate(with: desc, options: []) { (audiounit, error) in

            guard let au = audiounit else { return }

            self.engine.attach(au)
            let stereoFormat = AVAudioFormat(standardFormatWithSampleRate: format.sampleRate, channels: 2)

            // CRASHES HERE
            self.engine.connect(au, to: self.engine.mainMixerNode, format: stereoFormat)

            do {
              try self.engine.start()
            } catch (let exceptionError) {
                print(exceptionError)
            }
        }
    }
}

On the line that says "CRASHES HERE", I get an exception and see:

2017-07-22 14:20:39.208951-0400 AUInProcessTest[26638:7238083] [avae] AVAEInternal.h:98:_AVAE_CheckSuccessAndNoNSError: [AUInterface.mm:512:SetFormat: ([[busArray objectAtIndexedSubscript:(NSUInteger)element] setFormat:format error:&nsErr])] returned false, error (null) 2017-07-22 14:20:39.217199-0400 AUInProcessTest[26638:7238083] *** Terminating app due to uncaught exception 'com.apple.coreaudio.avfaudio', reason: '[[busArray objectAtIndexedSubscript:(NSUInteger)element] setFormat:format error:&nsErr]: returned false, error (null)'

I'm not sure what this means.