ArgumentOutOfRangeException Error in my game

I am trying to build a quiz game. I got two scripts, a Question Data and a Question Manager script. In my QuestionManager script i keep getting this error when i try to set a new question. I've tried to resolve it but i can't seem to get it fixed. Error ArgumentOutOfRangeException: Argument is out of range. Parameter name: index System.Collections.Generic.List`1[Question].get_Item (Int32 index) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/EqualityComparer.cs:84) QuestionManager.SetNewQuestion () (at Assets/QuestionManager.cs:30) QuestionManager.Start () (at Assets/QuestionManager.cs:21)

this is my QuestionManager script

 public class xxxxxxx {
 
     [SerializeField]
     private TextAsset questionDataXMLFile;
     public QuestionData questionData;
     private Question currentQuestion;
 
     [SerializeField]
     private Text factText;
     
     void Start() {
         questionData = QuestionData.LoadFromText(questionDataXMLFile.text);
         factText = GetComponent <Text>();
         SetNewQuestion ();
 
     }
 public void SetNewQuestion() 
 {
         int index = Random.Range(0, questionData.questions.Count - 1);
 
         Debug.Log (index);
 
         currentQuestion = questionData.questions[index]; 
 
         factText.text = currentQuestion.questionText
     }
     
     // Use this to see if user selected correct answer
     public bool CorrectAnswerSelected(int selectedAnswerID) {
         return selectedAnswerID == currentQuestion.correctAnswerID;
     }
 }