diff --git a/content/base/src/nsMappedAttributes.h b/content/base/src/nsMappedAttributes.h
index 7367779cd18..d402531d52e 100644
--- a/content/base/src/nsMappedAttributes.h
+++ b/content/base/src/nsMappedAttributes.h
@@ -108,6 +108,12 @@ public:
   virtual void List(FILE* out = stdout, PRInt32 aIndent = 0) const;
 #endif
 
+  PRInt64 SizeOf() const {
+    NS_ASSERTION(mAttrCount == mBufferSize,
+                 "mBufferSize and mAttrCount are expected to be the same.");
+    return sizeof(*this) - sizeof(void*) + mAttrCount * sizeof(InternalAttr);
+  }
+
 private:
   nsMappedAttributes(const nsMappedAttributes& aCopy);
   ~nsMappedAttributes();
diff --git a/layout/style/nsHTMLStyleSheet.cpp b/layout/style/nsHTMLStyleSheet.cpp
index e5c95faa211..6785a12a22b 100644
--- a/layout/style/nsHTMLStyleSheet.cpp
+++ b/layout/style/nsHTMLStyleSheet.cpp
@@ -540,7 +540,7 @@ GetHashEntryAttributesSize(PLDHashTable* aTable, PLDHashEntryHdr* aEntry,
   PRInt64 size = *static_cast<PRInt64*>(aArg);
 
   NS_ASSERTION(entry->mAttributes, "entry->mAttributes should not be null!");
-  size += sizeof(*entry->mAttributes);
+  size += entry->mAttributes->SizeOf();
 
   return PL_DHASH_NEXT;
 }