Using AVAudioUnitSampler as Audio Unit v3 Extension – distorted sound

Question

I have a music app that uses AVAudioUnitSampler to generate sounds. I'm trying to make an Audio Unit Extension, so my app will be able to be used in a host apps like Garage Band.

I implement AUViewController and return underlying AVAudioUnitSampler's audioUnit in createAudioUnit(with:) method like this:

public class AudioUnitViewController: AUViewController, AUAudioUnitFactory
{
    var sampler: AVAudioUnitSampler!

    public func createAudioUnit(with componentDescription: AudioComponentDescription) throws -> AUAudioUnit
    {
        //...
        //sampler loading code
        //...

        return sampler.auAudioUnit
    }
}

It somehow works (compiles and works without errors), but the sound I hear in GarageBand is very much distorted.

Is there anything I could change to use AVAudioUnitSampler as AU v3 extension?