diff --git a/layout/base/FrameLayerBuilder.cpp b/layout/base/FrameLayerBuilder.cpp
index 6db09c94ce4e..b78720e347c7 100644
--- a/layout/base/FrameLayerBuilder.cpp
+++ b/layout/base/FrameLayerBuilder.cpp
@@ -3109,7 +3109,7 @@ ContainerState::SetupMaskLayer(Layer *aLayer, const FrameLayerBuilder::Clip& aCl
  
   // check to see if we can reuse a mask image
   const MaskLayerImageCache::MaskLayerImageKey* key =
-    new MaskLayerImageCache::MaskLayerImageKey(roundedRects);
+    new MaskLayerImageCache::MaskLayerImageKey(roundedRects, aLayer->Manager()->GetBackendType());
   const MaskLayerImageCache::MaskLayerImageKey* lookupKey = key;
 
   nsRefPtr<ImageContainer> container =
diff --git a/layout/base/MaskLayerImageCache.h b/layout/base/MaskLayerImageCache.h
index 3dd54a530cfb..288aad2afa0e 100644
--- a/layout/base/MaskLayerImageCache.h
+++ b/layout/base/MaskLayerImageCache.h
@@ -113,8 +113,9 @@ public:
   class MaskLayerImageKey
   {
   public:
-    MaskLayerImageKey(const nsTArray<PixelRoundedRect>& aRoundedClipRects)
-      : mLayerCount(0)
+    MaskLayerImageKey(const nsTArray<PixelRoundedRect>& aRoundedClipRects, layers::LayersBackend aBackend)
+      : mBackend(aBackend)
+      , mLayerCount(0)
       , mRoundedClipRects(aRoundedClipRects)
     {}
 
@@ -132,6 +133,7 @@ public:
       for (PRUint32 i = 0; i < mRoundedClipRects.Length(); ++i) {
         hash = AddToHash(hash, mRoundedClipRects[i].Hash());
       }
+      hash = AddToHash(hash, mBackend);
 
       return hash;
     }
@@ -141,6 +143,7 @@ public:
       return mRoundedClipRects == aOther.mRoundedClipRects;
     }
 
+    layers::LayersBackend mBackend;
     mutable PRUint32 mLayerCount;
     nsTArray<PixelRoundedRect> mRoundedClipRects;
   };