Why is the text not complying with contour tag?

I got this SSML from w3. org. AVSpeechUtterance(ssmlRepresentation:) is not complying with the contour. It doesn't change hz.

    <?xml version="1.0"?>
    <speak version="1.1" xmlns="http://www.w3.org/2001/10/synthesis"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
                     http://www.w3.org/TR/speech-synthesis11/synthesis.xsd"
           xml:lang="en-US">
      <prosody contour="(0%,+20Hz) (10%,+30%) (40%,+10Hz)">
        good morning
      </prosody>
    </speak>

override func viewDidLoad() {
        super.viewDidLoad()

        guard let localUtterance =         AVSpeechUtterance(ssmlRepresentation: self.speechSML)         else {
            print("SML did not work.")
            return
        }
        self.utterance = localUtterance
        self.utterance.voice = self.voiceNoelle
}

self.synthesizer.speak(self.utterance)