diff --git a/reactos/subsystems/win32/win32k/ntuser/class.c b/reactos/subsystems/win32/win32k/ntuser/class.c
index 5e8f074bb41..03e4f5db839 100644
--- a/reactos/subsystems/win32/win32k/ntuser/class.c
+++ b/reactos/subsystems/win32/win32k/ntuser/class.c
@@ -385,6 +385,8 @@ IntSetClassWndProc(IN OUT PCLS Class,
       Class->Unicode = !Ansi;
    }
 
+   if (!WndProc) WndProc = Class->lpfnWndProc;
+
    chWndProc = WndProc;
 
    // Check if CallProc handle and retrieve previous call proc address and set.
diff --git a/reactos/subsystems/win32/win32k/ntuser/window.c b/reactos/subsystems/win32/win32k/ntuser/window.c
index 641db2cf99e..e561b709aee 100644
--- a/reactos/subsystems/win32/win32k/ntuser/window.c
+++ b/reactos/subsystems/win32/win32k/ntuser/window.c
@@ -663,6 +663,8 @@ IntSetWindowProc(PWND pWnd,
       if (pWnd->state & WNDS_SERVERSIDEWINDOWPROC)
          pWnd->state &= ~WNDS_SERVERSIDEWINDOWPROC;
 
+      if (!NewWndProc) NewWndProc = pWnd->lpfnWndProc;
+
       if (Class->fnid <= FNID_GHOST && Class->fnid >= FNID_BUTTON)
       {
          if (Ansi)