diff --git a/sc2/ChangeLog b/sc2/ChangeLog
index 28f74c971..e18250de8 100644
--- a/sc2/ChangeLog
+++ b/sc2/ChangeLog
@@ -1,4 +1,6 @@
 Changes towards version 0.4:
+- Patches to enforce the invariant that the GraphicsLock is held when 
+  SetFlashRect is called (bug #504) -Michael
 - Major refactoring of threadlib; see doc/devel/threads -Michael
 - Downgraded the GraphicsLock to an ordinary Mutex -Michael
 - Added movie player; only movies defined are intro and ending;
diff --git a/sc2/src/sc2code/gameopt.c b/sc2/src/sc2code/gameopt.c
index 3cba87165..8f53234f9 100644
--- a/sc2/src/sc2code/gameopt.c
+++ b/sc2/src/sc2code/gameopt.c
@@ -1014,7 +1014,9 @@ Restart:
 	}
 	else if (CurrentMenuState.cancel)
 	{
+		LockMutex (GraphicsLock);
 		SetFlashRect ((PRECT)~0L, (FRAME)0);
+		UnlockMutex (GraphicsLock);
 		pMS->ModuleFrame = 0;
 		pMS->CurState = (BYTE)pMS->delta_item;
 		ResumeMusic ();
diff --git a/sc2/src/sc2code/shipyard.c b/sc2/src/sc2code/shipyard.c
index 458f31761..38da9d81d 100644
--- a/sc2/src/sc2code/shipyard.c
+++ b/sc2/src/sc2code/shipyard.c
@@ -755,7 +755,9 @@ DoModifyShips (PMENU_STATE pMS)
 					else if (cancel)
 					{
 						pMS->delta_item ^= MODIFY_CREW_FLAG;
+						LockMutex (GraphicsLock);
 						SetFlashRect ((PRECT)~0L, (FRAME)0);
+						UnlockMutex (GraphicsLock);
 						DrawMenuStateStrings (PM_CREW, SHIPYARD_CREW);
 					}
 					else if (select)