diff --git a/src/Avalonia.ReactiveUI/AutoDataTemplateBindingHook.cs b/src/Avalonia.ReactiveUI/AutoDataTemplateBindingHook.cs
index fe4e9dd94a..b6cdb77f81 100644
--- a/src/Avalonia.ReactiveUI/AutoDataTemplateBindingHook.cs
+++ b/src/Avalonia.ReactiveUI/AutoDataTemplateBindingHook.cs
@@ -29,7 +29,7 @@ namespace Avalonia.ReactiveUI
 
         /// <inheritdoc/>
         public bool ExecuteHook(
-            object source, object target,
+            object? source, object target,
             Func<IObservedChange<object, object>[]> getCurrentViewModelProperties,
             Func<IObservedChange<object, object>[]> getCurrentViewProperties,
             BindingDirection direction)
diff --git a/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj b/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj
index 624c5772a9..2792ae1c91 100644
--- a/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj
+++ b/src/Avalonia.ReactiveUI/Avalonia.ReactiveUI.csproj
@@ -3,6 +3,8 @@
     <TargetFramework>netstandard2.0</TargetFramework>
     <PackageId>Avalonia.ReactiveUI</PackageId>
     <SignAssembly>false</SignAssembly>
+    <Nullable>enable</Nullable>
+    <WarningsAsErrors>nullable</WarningsAsErrors>
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\packages\Avalonia\Avalonia.csproj" />
diff --git a/src/Avalonia.ReactiveUI/ReactiveUserControl.cs b/src/Avalonia.ReactiveUI/ReactiveUserControl.cs
index f4824ade0a..4987349162 100644
--- a/src/Avalonia.ReactiveUI/ReactiveUserControl.cs
+++ b/src/Avalonia.ReactiveUI/ReactiveUserControl.cs
@@ -17,8 +17,8 @@ namespace Avalonia.ReactiveUI
     /// <typeparam name="TViewModel">ViewModel type.</typeparam>
     public class ReactiveUserControl<TViewModel> : UserControl, IViewFor<TViewModel> where TViewModel : class
     {
-        public static readonly StyledProperty<TViewModel> ViewModelProperty = AvaloniaProperty
-            .Register<ReactiveUserControl<TViewModel>, TViewModel>(nameof(ViewModel));
+        public static readonly StyledProperty<TViewModel?> ViewModelProperty = AvaloniaProperty
+            .Register<ReactiveUserControl<TViewModel>, TViewModel?>(nameof(ViewModel));
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ReactiveUserControl{TViewModel}"/> class.
@@ -34,16 +34,16 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// The ViewModel.
         /// </summary>
-        public TViewModel ViewModel
+        public TViewModel? ViewModel
         {
             get => GetValue(ViewModelProperty);
             set => SetValue(ViewModelProperty, value);
         }
 
-        object IViewFor.ViewModel
+        object? IViewFor.ViewModel
         {
             get => ViewModel;
-            set => ViewModel = (TViewModel)value;
+            set => ViewModel = (TViewModel?)value;
         }
 
         protected override void OnDataContextChanged(EventArgs e)
@@ -51,7 +51,7 @@ namespace Avalonia.ReactiveUI
             ViewModel = DataContext as TViewModel;
         }
 
-        private void OnViewModelChanged(object value)
+        private void OnViewModelChanged(object? value)
         {
             if (value == null)
             {
diff --git a/src/Avalonia.ReactiveUI/ReactiveWindow.cs b/src/Avalonia.ReactiveUI/ReactiveWindow.cs
index 758a807bfc..726fb3d661 100644
--- a/src/Avalonia.ReactiveUI/ReactiveWindow.cs
+++ b/src/Avalonia.ReactiveUI/ReactiveWindow.cs
@@ -17,8 +17,8 @@ namespace Avalonia.ReactiveUI
     /// <typeparam name="TViewModel">ViewModel type.</typeparam>
     public class ReactiveWindow<TViewModel> : Window, IViewFor<TViewModel> where TViewModel : class
     {
-        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.
@@ -35,19 +35,19 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// The ViewModel.
         /// </summary>
-        public TViewModel ViewModel
+        public TViewModel? ViewModel
         {
             get => GetValue(ViewModelProperty);
             set => SetValue(ViewModelProperty, value);
         }
 
-        object IViewFor.ViewModel
+        object? IViewFor.ViewModel
         {
             get => ViewModel;
-            set => ViewModel = (TViewModel)value;
+            set => ViewModel = (TViewModel?)value;
         }
 
-        private void OnDataContextChanged(object value)
+        private void OnDataContextChanged(object? value)
         {
             if (value is TViewModel viewModel)
             {
@@ -59,7 +59,7 @@ namespace Avalonia.ReactiveUI
             }
         }
 
-        private void OnViewModelChanged(object value)
+        private void OnViewModelChanged(object? value)
         {
             if (value == null)
             {
diff --git a/src/Avalonia.ReactiveUI/RoutedViewHost.cs b/src/Avalonia.ReactiveUI/RoutedViewHost.cs
index 421633cd58..63456bc13a 100644
--- a/src/Avalonia.ReactiveUI/RoutedViewHost.cs
+++ b/src/Avalonia.ReactiveUI/RoutedViewHost.cs
@@ -55,8 +55,8 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// <see cref="AvaloniaProperty"/> for the <see cref="Router"/> property.
         /// </summary>
-        public static readonly StyledProperty<RoutingState> RouterProperty =
-            AvaloniaProperty.Register<RoutedViewHost, RoutingState>(nameof(Router));
+        public static readonly StyledProperty<RoutingState?> RouterProperty =
+            AvaloniaProperty.Register<RoutedViewHost, RoutingState?>(nameof(Router));
     
         /// <summary>
         /// Initializes a new instance of the <see cref="RoutedViewHost"/> class.
@@ -67,12 +67,12 @@ namespace Avalonia.ReactiveUI
             {
                 var routerRemoved = this
                     .WhenAnyValue(x => x.Router)
-                    .Where(router => router == null)
-                    .Cast<object>();
+                    .Where(router => router == null)!
+                    .Cast<object?>();
 
                 this.WhenAnyValue(x => x.Router)
                     .Where(router => router != null)
-                    .SelectMany(router => router.CurrentViewModel)
+                    .SelectMany(router => router!.CurrentViewModel)
                     .Merge(routerRemoved)
                     .Subscribe(NavigateToViewModel)
                     .DisposeWith(disposables);
@@ -82,7 +82,7 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the <see cref="RoutingState"/> of the view model stack.
         /// </summary>
-        public RoutingState Router
+        public RoutingState? Router
         {
             get => GetValue(RouterProperty);
             set => SetValue(RouterProperty, value);
@@ -91,13 +91,13 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the ReactiveUI view locator used by this router.
         /// </summary>
-        public IViewLocator ViewLocator { get; set; }
+        public IViewLocator? ViewLocator { get; set; }
     
         /// <summary>
         /// Invoked when ReactiveUI router navigates to a view model.
         /// </summary>
         /// <param name="viewModel">ViewModel to which the user navigates.</param>
-        private void NavigateToViewModel(object viewModel)
+        private void NavigateToViewModel(object? viewModel)
         {
             if (Router == null)
             {
diff --git a/src/Avalonia.ReactiveUI/TransitioningContentControl.cs b/src/Avalonia.ReactiveUI/TransitioningContentControl.cs
index cc1613ce19..9685ecbe91 100644
--- a/src/Avalonia.ReactiveUI/TransitioningContentControl.cs
+++ b/src/Avalonia.ReactiveUI/TransitioningContentControl.cs
@@ -13,20 +13,20 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// <see cref="AvaloniaProperty"/> for the <see cref="PageTransition"/> property.
         /// </summary>
-        public static readonly StyledProperty<IPageTransition> PageTransitionProperty =
-            AvaloniaProperty.Register<TransitioningContentControl, IPageTransition>(nameof(PageTransition),
+        public static readonly StyledProperty<IPageTransition?> PageTransitionProperty =
+            AvaloniaProperty.Register<TransitioningContentControl, IPageTransition?>(nameof(PageTransition),
                 new CrossFade(TimeSpan.FromSeconds(0.5)));
 
         /// <summary>
         /// <see cref="AvaloniaProperty"/> for the <see cref="DefaultContent"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> DefaultContentProperty =
-            AvaloniaProperty.Register<TransitioningContentControl, object>(nameof(DefaultContent));
+        public static readonly StyledProperty<object?> DefaultContentProperty =
+            AvaloniaProperty.Register<TransitioningContentControl, object?>(nameof(DefaultContent));
         
         /// <summary>
         /// Gets or sets the animation played when content appears and disappears.
         /// </summary>
-        public IPageTransition PageTransition
+        public IPageTransition? PageTransition
         {
             get => GetValue(PageTransitionProperty);
             set => SetValue(PageTransitionProperty, value);
@@ -35,7 +35,7 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the content displayed whenever there is no page currently routed.
         /// </summary>
-        public object DefaultContent
+        public object? DefaultContent
         {
             get => GetValue(DefaultContentProperty);
             set => SetValue(DefaultContentProperty, value);
@@ -44,7 +44,7 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the content with animation.
         /// </summary>
-        public new object Content
+        public new object? Content
         {
             get => base.Content;
             set => UpdateContentWithTransition(value);
@@ -60,7 +60,7 @@ namespace Avalonia.ReactiveUI
         /// Updates the content with transitions.
         /// </summary>
         /// <param name="content">New content to set.</param>
-        private async void UpdateContentWithTransition(object content)
+        private async void UpdateContentWithTransition(object? content)
         {
             if (PageTransition != null)
                 await PageTransition.Start(this, null, true);
diff --git a/src/Avalonia.ReactiveUI/ViewModelViewHost.cs b/src/Avalonia.ReactiveUI/ViewModelViewHost.cs
index 0169b268ca..c88323d674 100644
--- a/src/Avalonia.ReactiveUI/ViewModelViewHost.cs
+++ b/src/Avalonia.ReactiveUI/ViewModelViewHost.cs
@@ -15,8 +15,8 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// <see cref="AvaloniaProperty"/> for the <see cref="ViewModel"/> property.
         /// </summary>
-        public static readonly AvaloniaProperty<object> ViewModelProperty =
-            AvaloniaProperty.Register<ViewModelViewHost, object>(nameof(ViewModel));
+        public static readonly AvaloniaProperty<object?> ViewModelProperty =
+            AvaloniaProperty.Register<ViewModelViewHost, object?>(nameof(ViewModel));
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ViewModelViewHost"/> class.
@@ -34,7 +34,7 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the ViewModel to display.
         /// </summary>
-        public object ViewModel
+        public object? ViewModel
         {
             get => GetValue(ViewModelProperty);
             set => SetValue(ViewModelProperty, value);
@@ -43,13 +43,13 @@ namespace Avalonia.ReactiveUI
         /// <summary>
         /// Gets or sets the view locator.
         /// </summary>
-        public IViewLocator ViewLocator { get; set; }
+        public IViewLocator? ViewLocator { get; set; }
 
         /// <summary>
         /// Invoked when ReactiveUI router navigates to a view model.
         /// </summary>
         /// <param name="viewModel">ViewModel to which the user navigates.</param>
-        private void NavigateToViewModel(object viewModel)
+        private void NavigateToViewModel(object? viewModel)
         {
             if (viewModel == null)
             {
@@ -74,4 +74,4 @@ namespace Avalonia.ReactiveUI
             Content = viewInstance;
         }
     }
-}
\ No newline at end of file
+}