diff --git a/backends/sdl/sdl-common.cpp b/backends/sdl/sdl-common.cpp
index 28e87db04b8..3a261026c68 100644
--- a/backends/sdl/sdl-common.cpp
+++ b/backends/sdl/sdl-common.cpp
@@ -148,7 +148,7 @@ void OSystem_SDL_Common::copy_rect(const byte *buf, int pitch, int x, int y, int
 	if (_screen == NULL)
 		return;
 
-	StackLock lock(_graphicsMutex);	// Lock the mutex until this function ends
+	StackLock lock(_graphicsMutex, this);	// Lock the mutex until this function ends
 	
 	if (((uint32)buf & 3) == 0 && pitch == _screenWidth && x==0 && y==0 &&
 			w==_screenWidth && h==_screenHeight && _mode_flags&DF_WANT_RECT_OPTIM) {
@@ -1229,7 +1229,7 @@ void OSystem_SDL_Common::clear_overlay() {
 	if (!_overlayVisible)
 		return;
 	
-	StackLock lock(_graphicsMutex);	// Lock the mutex until this function ends
+	StackLock lock(_graphicsMutex, this);	// Lock the mutex until this function ends
 	
 	// hide the mouse
 	undraw_mouse();
diff --git a/backends/sdl/sdl.cpp b/backends/sdl/sdl.cpp
index 0e2d2b42a11..8d73295be4e 100644
--- a/backends/sdl/sdl.cpp
+++ b/backends/sdl/sdl.cpp
@@ -210,7 +210,7 @@ void OSystem_SDL::hotswap_gfx_mode() {
 void OSystem_SDL::update_screen() {
 	assert(_hwscreen != NULL);
 
-	StackLock lock(_graphicsMutex);	// Lock the mutex until this function ends
+	StackLock lock(_graphicsMutex, this);	// Lock the mutex until this function ends
 
 	// If the shake position changed, fill the dirty area with blackness
 	if (_currentShakePos != _newShakePos) {
@@ -336,7 +336,7 @@ void OSystem_SDL::update_screen() {
 
 uint32 OSystem_SDL::property(int param, Property *value) {
 
-	StackLock lock(_graphicsMutex);	// Lock the mutex until this function ends
+	StackLock lock(_graphicsMutex, this);	// Lock the mutex until this function ends
 
 	if (param == PROP_TOGGLE_FULLSCREEN) {
 		assert(_hwscreen != 0);
diff --git a/backends/sdl/sdl_gl.cpp b/backends/sdl/sdl_gl.cpp
index b671601494e..a7bea1c9f0a 100644
--- a/backends/sdl/sdl_gl.cpp
+++ b/backends/sdl/sdl_gl.cpp
@@ -348,7 +348,7 @@ void OSystem_SDL_OpenGL::hotswap_gfx_mode() {
 
 void OSystem_SDL_OpenGL::update_screen() {
 
-	StackLock lock(_graphicsMutex);	// Lock the mutex until this function ends
+	StackLock lock(_graphicsMutex, this);	// Lock the mutex until this function ends
 
 	// If the shake position changed, fill the dirty area with blackness
 	if (_currentShakePos != _newShakePos) {
@@ -548,7 +548,7 @@ bool OSystem_SDL_OpenGL::poll_event(Event *event) {
 
 uint32 OSystem_SDL_OpenGL::property(int param, Property *value) {
 
-	StackLock lock(_graphicsMutex); // Lock the mutex until this function ends
+	StackLock lock(_graphicsMutex, this); // Lock the mutex until this function ends
 
 	if (param == PROP_TOGGLE_FULLSCREEN) {
 		if (!_usingOpenGL)
diff --git a/common/util.cpp b/common/util.cpp
index 548fcb6a419..8920453bd7b 100644
--- a/common/util.cpp
+++ b/common/util.cpp
@@ -166,7 +166,10 @@ uint RandomSource::getRandomNumberRng(uint min, uint max) {
 	return getRandomNumber(max - min) + min;
 }
 
-StackLock::StackLock(OSystem::MutexRef mutex) : _mutex(mutex) {
+StackLock::StackLock(OSystem::MutexRef mutex, OSystem *syst)
+	: _mutex(mutex), _syst(syst) {
+	if (syst == 0)
+		_syst = g_system;
 	lock();
 }
 
@@ -175,12 +178,12 @@ StackLock::~StackLock() {
 }
 
 void StackLock::lock() {
-	assert(g_system);
+	assert(_syst);
 	g_system->lock_mutex(_mutex);
 }
 
 void StackLock::unlock() {
-	assert(g_system);
+	assert(_syst);
 	g_system->unlock_mutex(_mutex);
 }
 
diff --git a/common/util.h b/common/util.h
index 8c0409e0ef6..3a774455675 100644
--- a/common/util.h
+++ b/common/util.h
@@ -80,10 +80,11 @@ public:
  */
 class StackLock {
 	OSystem::MutexRef _mutex;
+	OSystem *_syst;
 	void lock();
 	void unlock();
 public:
-	StackLock(OSystem::MutexRef mutex);
+	StackLock(OSystem::MutexRef mutex, OSystem *syst = 0);
 	~StackLock();
 };