diff --git a/.travis.yml b/.travis.yml
index 58e4769..25a651f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -23,11 +23,11 @@
   - make
   - ./unittests
 
-notifications:
-  email:
-    recipients:
-      - swiftshader-team@google.com
-    on_success: never
+notifications:
+  email:
+    recipients:
+      - swiftshader-team@google.com
+    on_success: never
     on_failure: always
 
 deploy:
diff --git a/src/Common/Resource.cpp b/src/Common/Resource.cpp
index 0e71e05..3a63810 100644
--- a/src/Common/Resource.cpp
+++ b/src/Common/Resource.cpp
@@ -15,6 +15,7 @@
 #include "Resource.hpp"
 
 #include "Memory.hpp"
+#include "Debug.hpp"
 
 namespace sw
 {
@@ -106,6 +107,7 @@
 	void Resource::unlock()
 	{
 		criticalSection.lock();
+		ASSERT(count > 0);
 
 		count--;
 
@@ -131,6 +133,7 @@
 	void Resource::unlock(Accessor relinquisher)
 	{
 		criticalSection.lock();
+		ASSERT(count > 0);
 
 		while(count > 0 && accessor == relinquisher)
 		{
diff --git a/src/Renderer/Surface.cpp b/src/Renderer/Surface.cpp
index 9c47f1c..e06f2bd 100644
--- a/src/Renderer/Surface.cpp
+++ b/src/Renderer/Surface.cpp
@@ -1389,9 +1389,9 @@
 
 		deallocate(stencil.buffer);
 
-		external.buffer = 0;
-		internal.buffer = 0;
-		stencil.buffer = 0;
+		external.buffer = nullptr;
+		internal.buffer = nullptr;
+		stencil.buffer = nullptr;
 	}
 
 	void *Surface::lockExternal(int x, int y, int z, Lock lock, Accessor client)
@@ -1529,13 +1529,13 @@
 
 	void *Surface::lockStencil(int x, int y, int front, Accessor client)
 	{
+		resource->lock(client);
+
 		if(stencil.format == FORMAT_NULL)
 		{
 			return nullptr;
 		}
 
-		resource->lock(client);
-
 		if(!stencil.buffer)
 		{
 			stencil.buffer = allocateBuffer(stencil.width, stencil.height, stencil.depth, stencil.border, stencil.samples, stencil.format);