diff --git a/programs/winhlp32/winhelp.c b/programs/winhlp32/winhelp.c
index 7cb4a12..7325420 100644
--- a/programs/winhlp32/winhelp.c
+++ b/programs/winhlp32/winhelp.c
@@ -1594,7 +1594,7 @@ static BOOL WINHELP_RegisterWinClasses(void)
     class_main.style               = CS_HREDRAW | CS_VREDRAW;
     class_main.lpfnWndProc         = WINHELP_MainWndProc;
     class_main.cbClsExtra          = 0;
-    class_main.cbWndExtra          = sizeof(LONG);
+    class_main.cbWndExtra          = sizeof(WINHELP_WINDOW *);
     class_main.hInstance           = Globals.hInstance;
     class_main.hIcon               = LoadIcon(Globals.hInstance, MAKEINTRESOURCE(IDI_WINHELP));
     class_main.hCursor             = LoadCursor(0, IDC_ARROW);