First of all thank you all for downvoting me, it really improves the chances of me leaving an answer. In my case it was a Flutter custom plugin issue, the problem was solved by changing: func speak(_ text: String) { textToSpeech.speak(text) } by func speak(_ text: String, result: @escaping FlutterResult) { textToSpeech.speak(text, result: result) }

Problem solved. I was returning the wrong type in a method within the context of my code's logic.