diff --git a/Pinta.Tools/Tools/GradientTool.cs b/Pinta.Tools/Tools/GradientTool.cs
index a3bb9f14f..d496e05cf 100644
--- a/Pinta.Tools/Tools/GradientTool.cs
+++ b/Pinta.Tools/Tools/GradientTool.cs
@@ -35,7 +35,7 @@ public sealed class GradientTool : BaseTool
 		private readonly IPaletteService palette;
 		PointD startpoint;
 		bool tracking;
-		protected ImageSurface? undo_surface;
+		private ImageSurface? undo_surface;
 		MouseButton button;
 
 		private const string GRADIENT_TYPE_SETTING = "gradient-type";