diff --git a/src/Canvas.cc b/src/Canvas.cc
index b7fee0c..cd3dedb 100644
--- a/src/Canvas.cc
+++ b/src/Canvas.cc
@@ -294,6 +294,7 @@ Canvas::Canvas(int w, int h): ObjectWrap() {
   width = w;
   height = h;
   _surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, w, h);
+  V8::AdjustAmountOfExternalAllocatedMemory(4 * width * height);
 }
 
 /*
@@ -311,9 +312,11 @@ Canvas::~Canvas() {
 void
 Canvas::resurface(Handle<Object> canvas) {
   // Re-surface
+  int old_width = cairo_image_surface_get_width(_surface);
+  int old_height = cairo_image_surface_get_height(_surface);
   cairo_surface_destroy(_surface);
   _surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, width, height);
-  V8::AdjustAmountOfExternalAllocatedMemory(-4 * width * height);
+  V8::AdjustAmountOfExternalAllocatedMemory(4 * (width * height - old_width * old_height));
 
   // Reset context
   Handle<Value> context = canvas->Get(String::New("context"));