AudioKit crash when mixing Balancers

Ok so I got this weird crash on iOS when trying to use AKBalancer in an AKMixer with an AKPlayer.

[avae] AVAEInternal.h:70:_AVAE_Check: required condition is false: [ (mixingDest)]

1) Initially, I do this during initialisation:

let mixer = AKMixer(self.MixerNodes)
AudioKit.output = mixer
try AudioKit.start()

where MixerNodes is an empty pool.

2) Then I set it to play an audio track.

//create player
player = AKPlayer(audioFile: audioFile)

//create balancer
let highPass = AKHighPassFilter(player, cutoffFrequency: 900)
let lowPass = AKLowPassFilter(highPass, cutoffFrequency: 300)
balancer = AKBalancer(lowPass, comparator: player)

//create mixer and store it somewhere
let mixer = AKMixer(player,balancer)

//reset MixerNodes pool

//for each player & balancer pair that currently exist
//add to MixerNodes pool

//re-create output mixer
let mixer = AKMixer(self.MixerNodes)

//set mixer to audio output
AudioKit.output = mixer

//play audio file

3) The balancer works fine for the initial audio track but when I inject a second (player, balancer) pair into the output mixer, the crash will happen. Does anyone know where I did wrongly?