diff --git a/reactos/win32ss/gdi/eng/bitblt.c b/reactos/win32ss/gdi/eng/bitblt.c
index dfb61337925..a956cbf4530 100644
--- a/reactos/win32ss/gdi/eng/bitblt.c
+++ b/reactos/win32ss/gdi/eng/bitblt.c
@@ -50,6 +50,8 @@ BltMask(SURFOBJ* psoDest,
 
     ASSERT(IS_VALID_ROP4(Rop4));
 
+    if (!psoMask) return FALSE;
+
     fgndRop = ROP4_FGND(Rop4);
     bkgndRop = ROP4_BKGND(Rop4);