How is scheduleParameterBlock supposed to work from a host?

I'm having no luck trying to get scheduleParameterBlock working on the gain of first input AVAudioEngine main mixer node. It just doesn't do anything. Am I missing something here?

// CLASS VARS
var inputGain : AUParameter?
var mixerParameterBlock : AUScheduleParameterBlock?
let rampTime :AUAudioFrameCount = AUAudioFrameCount(10 * 44100) // ramp over 10 seconds



// IN PLAY FUNCTION
// set gain on input of mixer which will be the first parameter in the tree
let parameterTree = engine.mainMixerNode.auAudioUnit.parameterTree
inputGain = (parameterTree?.parameter(withAddress: 0x2000000000000000))!

// set initial gain to 1.0
inputGain!.value = 1.0

// reduces gain after 1 second delay
// THIS WORKS
delay(1.0) {
    print("reducing gain to 0.5")
    self.inputGain!.value = 0.5
}

if (inputGain?.flags.contains(.flag_CanRamp))! {
    // inputGain says it is rampable
    print("inputGain parameter is rampable")
}

mixerParameterBlock = engine.mainMixerNode.auAudioUnit.scheduleParameterBlock

// THIS DOES NOT WORK
mixerParameterBlock!(AUEventSampleTimeImmediate, rampTime, inputGain!.address, 0.0)

player?.play()
player?.scheduleFile(audioFile, at: nil)