C# convert enum? to int? fail when using reflection

when using (int?) to convert, It succeed； when using reflection to convert, It failed; How can I assign value(enum?) to property(int?) success using reflection?

static void Main(string[] args)
        {
            Dc dc = new Dc { Solution = Solution.Upgrade };

            Model model = new Model {  };

            //assign by reflection
            var psolution = model.GetType().GetProperty("Solution");
            //psolution.SetValue(model, dc.Solution); //this fail
            model.Solution = (int?)dc.Solution; //this success
            psolution.SetValue(model, Convert.ChangeType(dc.Solution, psolution.PropertyType)); //this fail
        }

        class Dc
        {
            public Solution? Solution { get; set; }
        }

        class Model
        {
            public int? Solution { get; set; }
        }

        enum Solution
        {
            Upgrade = 1,
            Discard = 2,
        }