diff --git a/src/Avalonia.Controls/ToolTip.cs b/src/Avalonia.Controls/ToolTip.cs
index 71bd0726d4..ab507d07a2 100644
--- a/src/Avalonia.Controls/ToolTip.cs
+++ b/src/Avalonia.Controls/ToolTip.cs
@@ -1,3 +1,4 @@
+#nullable enable
using System;
using System.Reactive.Linq;
using Avalonia.Controls.Metadata;
@@ -21,8 +22,8 @@ namespace Avalonia.Controls
///
/// Defines the ToolTip.Tip attached property.
///
- public static readonly AttachedProperty