diff --git a/reactos/dll/win32/user32/windows/message.c b/reactos/dll/win32/user32/windows/message.c
index f4bf39cde4d..78794293b0c 100644
--- a/reactos/dll/win32/user32/windows/message.c
+++ b/reactos/dll/win32/user32/windows/message.c
@@ -1141,6 +1141,12 @@ IntCallWindowProcW(BOOL IsAnsiProc,
   MSG UnicodeMsg;
   LRESULT Result;
 
+  if (WndProc == NULL)
+  {
+      DPRINT1("IntCallWindowsProcW() called with WndProc = NULL!\n");
+      return FALSE;
+  }
+
   if (IsAnsiProc)
     {
       UnicodeMsg.hwnd = hWnd;
@@ -1177,6 +1183,12 @@ IntCallWindowProcA(BOOL IsAnsiProc,
   MSG UnicodeMsg;
   LRESULT Result;
 
+  if (WndProc == NULL)
+  {
+      DPRINT1("IntCallWindowsProcA() called with WndProc = NULL!\n");
+      return FALSE;
+  }
+
   if (IsAnsiProc)
     {
       return WndProc(hWnd, Msg, wParam, lParam);