diff --git a/src/gui/window.c b/src/gui/window.c
index aa26d0ab..85e1676c 100644
--- a/src/gui/window.c
+++ b/src/gui/window.c
@@ -138,7 +138,7 @@ static void RecalcClientArea (HWND hWnd)
 
             if (pWin->secondaryDC == HDC_INVALID) {
                 /* remove the flag of WS_EX_AUTOSECONDARYDC */
-                pWin->dwExStyle = pWin->dwExStyle | WS_EX_AUTOSECONDARYDC;
+                pWin->dwExStyle &= ~WS_EX_AUTOSECONDARYDC;
                 pWin->secondaryDC = 0;
             }
 
diff --git a/src/kernel/desktop-comm.c b/src/kernel/desktop-comm.c
index 9e742d6e..f170c761 100644
--- a/src/kernel/desktop-comm.c
+++ b/src/kernel/desktop-comm.c
@@ -232,7 +232,7 @@ static int dskAddNewMainWindow (PMAINWIN pWin)
 
     if (pWin->secondaryDC == HDC_INVALID) {
         /* remove the flag of WS_EX_AUTOSECONDARYDC */
-        pWin->dwExStyle = pWin->dwExStyle | WS_EX_AUTOSECONDARYDC;
+        pWin->dwExStyle &= ~WS_EX_AUTOSECONDARYDC;
         pWin->secondaryDC = 0;
 
     }