diff --git a/DragonFruit.Kaplan/ReactiveAppWindow.cs b/DragonFruit.Kaplan/ReactiveAppWindow.cs
index 4c4f80c..c667cf4 100644
--- a/DragonFruit.Kaplan/ReactiveAppWindow.cs
+++ b/DragonFruit.Kaplan/ReactiveAppWindow.cs
@@ -7,6 +7,8 @@
 using FluentAvalonia.UI.Windowing;
 using ReactiveUI;
 
+#nullable enable
+
 namespace DragonFruit.Kaplan
 {
     /// <summary>
@@ -23,7 +25,7 @@ namespace DragonFruit.Kaplan
     public class ReactiveAppWindow<TViewModel> : AppWindow, IViewFor<TViewModel> where TViewModel : class
     {
         [System.Diagnostics.CodeAnalysis.SuppressMessage("AvaloniaProperty", "AVP1002", Justification = "Generic avalonia property is expected here.")]
-        public static readonly StyledProperty<TViewModel> ViewModelProperty = AvaloniaProperty.Register<ReactiveWindow<TViewModel>, TViewModel?>(nameof(ViewModel));
+        public static readonly StyledProperty<TViewModel?> ViewModelProperty = AvaloniaProperty.Register<ReactiveWindow<TViewModel>, TViewModel?>(nameof(ViewModel));
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ReactiveWindow{TViewModel}"/> class.