iOS 14 AUGraphInitialize an error code -10877

Hello.

When trying to add more than 8 nodes to the graph, the AUGraphInitialize function returns an error code -10877 (kAudioUnitErr_InvalidElement).

What is wrong? How can I debug this InvalidElement?

var acd: AudioComponentDescription = AudioComponentDescription()

acd.componentManufacturer = kAudioUnitManufacturer_Apple
acd.componentType = kAudioUnitType_MusicDevice
acd.componentSubType = kAudioUnitSubType_MIDISynth

var units = []

for i in 0..<number {
    var instrumentNode = AUNode()
    AUGraphAddNode(graph, &acd, &instrumentNode)

    var instrumentUnit: AudioUnit!
    AUGraphNodeInfo(graph, instrumentNode, nil, &instrumentUnit)
    AUGraphConnectNodeInput(graph, instrumentNode, 0, mixerNode, UInt32(i))
        
    units.append(instrumentUnit)
}

AUGraphConnectNodeInput(graph, mixerNode, 0, outNode, 0)
AUGraphInitialize(graph)
AUGraphStart(graph)