diff --git a/reactos/dll/win32/user32/windows/mdi.c b/reactos/dll/win32/user32/windows/mdi.c
index 4d0e8f58e41..87e5adaa1c0 100644
--- a/reactos/dll/win32/user32/windows/mdi.c
+++ b/reactos/dll/win32/user32/windows/mdi.c
@@ -305,7 +305,7 @@ void MDI_CalcDefaultChildPos( HWND hwndClient, INT total, LPPOINT lpPos, INT del
     {
         MDICLIENTINFO *ci = get_client_info(hwndClient);
         total = ci ? ci->nTotalCreated : 0;
-        *id = ci->idFirstChild + ci->nActiveChildren;
+        *id = ci ? ci->idFirstChild + ci->nActiveChildren : 0;
         TRACE("MDI child id %04x\n", *id);
     }