Set indexed value by reflection gives me TargetParameterCountException

I have a PolygonRenderer class containing a Vertices property, which is a List, holding the points of the polygon the class renders.

When I try to change a specific point in this list by reflection, I get a System.Reflection.TargetParameterCountException on the last line of my function :

    public override void ApplyValue(string property, object value, int? index)
    {
        List<PropertyInfo> properties = Data.GetType().GetProperties().ToList();
        PropertyInfo pi = properties.FirstOrDefault(p => p.Name == property);
        pi.SetValue(Data, value,
            index.HasValue ? new object[] { index.Value } : null);
    }

When I debug, I get index.Value = 3, Data is the PolygonRenderer instance and pi reflects the Vertices property, which count = 4.

Since my index is supposed to be the last item of my list, how is it possible that I get a count exception on that property ?

Thanks