diff --git a/Libraries/LibGUI/Forward.h b/Libraries/LibGUI/Forward.h
index fda60a42d1..43bbd7a217 100644
--- a/Libraries/LibGUI/Forward.h
+++ b/Libraries/LibGUI/Forward.h
@@ -42,6 +42,7 @@ class FileSystemModel;
 class Frame;
 class GroupBox;
 class HorizontalBoxLayout;
+class HorizontalSlider;
 class IconView;
 class JsonArrayModel;
 class KeyEvent;
@@ -79,6 +80,7 @@ class ToolBar;
 class ToolBarContainer;
 class Variant;
 class VerticalBoxLayout;
+class VerticalSlider;
 class WMEvent;
 class Widget;
 class Window;