diff --git a/layout/style/nsCSSValue.cpp b/layout/style/nsCSSValue.cpp
index bc42a7e209b..c95869203d5 100644
--- a/layout/style/nsCSSValue.cpp
+++ b/layout/style/nsCSSValue.cpp
@@ -2480,18 +2480,9 @@ css::ImageValue::ImageValue(nsIURI* aURI, nsStringBuffer* aString,
 css::ImageValue::~ImageValue()
 {
   for (auto iter = mRequests.Iter(); !iter.Done(); iter.Next()) {
-    nsISupports* key = iter.Key();
+    nsIDocument* doc = iter.Key();
     RefPtr<imgRequestProxy>& proxy = iter.Data();
 
-    nsIDocument* doc = static_cast<nsIDocument*>(key);
-
-#ifdef DEBUG
-    {
-      nsCOMPtr<nsIDocument> slowDoc = do_QueryInterface(key);
-      MOZ_ASSERT(slowDoc == doc);
-    }
-#endif
-
     if (doc) {
       doc->StyleImageLoader()->DeregisterCSSImage(this);
     }
diff --git a/layout/style/nsCSSValue.h b/layout/style/nsCSSValue.h
index 6c27d6e2ccb..1d912fa5be6 100644
--- a/layout/style/nsCSSValue.h
+++ b/layout/style/nsCSSValue.h
@@ -147,7 +147,7 @@ private:
 public:
   // Inherit operator== from URLValue
 
-  nsRefPtrHashtable<nsPtrHashKey<nsISupports>, imgRequestProxy> mRequests; 
+  nsRefPtrHashtable<nsPtrHashKey<nsIDocument>, imgRequestProxy> mRequests;
 
   // Override AddRef and Release to not only log ourselves correctly, but
   // also so that we delete correctly without a virtual destructor (assuming