diff --git a/dom/base/nsNameSpaceManager.cpp b/dom/base/nsNameSpaceManager.cpp
index 4a1c2c7b5013..1ce7b3ff73ef 100644
--- a/dom/base/nsNameSpaceManager.cpp
+++ b/dom/base/nsNameSpaceManager.cpp
@@ -66,6 +66,8 @@ bool nsNameSpaceManager::Init()
 
 
   // Need to be ordered according to ID.
+  MOZ_ASSERT(mURIArray.IsEmpty());
+  REGISTER_NAMESPACE(nsGkAtoms::empty, kNameSpaceID_None);
   REGISTER_NAMESPACE(nsGkAtoms::nsuri_xmlns, kNameSpaceID_XMLNS);
   REGISTER_NAMESPACE(nsGkAtoms::nsuri_xml, kNameSpaceID_XML);
   REGISTER_NAMESPACE(nsGkAtoms::nsuri_xhtml, kNameSpaceID_XHTML);
@@ -97,7 +99,7 @@ nsNameSpaceManager::RegisterNameSpace(const nsAString& aURI,
   nsCOMPtr<nsIAtom> atom = NS_Atomize(aURI);
   nsresult rv = NS_OK;
   if (!mURIToIDTable.Get(atom, &aNameSpaceID)) {
-    aNameSpaceID = mURIArray.Length() + 1; // id is index + 1
+    aNameSpaceID = mURIArray.Length();
 
     rv = AddNameSpace(atom.forget(), aNameSpaceID);
     if (NS_FAILED(rv)) {
@@ -114,15 +116,16 @@ nsresult
 nsNameSpaceManager::GetNameSpaceURI(int32_t aNameSpaceID, nsAString& aURI)
 {
   NS_PRECONDITION(aNameSpaceID >= 0, "Bogus namespace ID");
-  
-  int32_t index = aNameSpaceID - 1; // id is index + 1
-  if (index < 0 || index >= int32_t(mURIArray.Length())) {
+
+  // We have historically treated GetNameSpaceURI calls for kNameSpaceID_None
+  // as erroneous.
+  if (aNameSpaceID <= 0 || aNameSpaceID >= int32_t(mURIArray.Length())) {
     aURI.Truncate();
 
     return NS_ERROR_ILLEGAL_VALUE;
   }
 
-  mURIArray.ElementAt(index)->ToString(aURI);
+  mURIArray.ElementAt(aNameSpaceID)->ToString(aURI);
 
   return NS_OK;
 }
@@ -224,10 +227,8 @@ nsresult nsNameSpaceManager::AddNameSpace(already_AddRefed<nsIAtom> aURI,
     // We've wrapped...  Can't do anything else here; just bail.
     return NS_ERROR_OUT_OF_MEMORY;
   }
-  
-  NS_ASSERTION(aNameSpaceID - 1 == (int32_t) mURIArray.Length(),
-               "BAD! AddNameSpace not called in right order!");
 
+  MOZ_ASSERT(aNameSpaceID == (int32_t) mURIArray.Length());
   mURIArray.AppendElement(uri.forget());
   mURIToIDTable.Put(mURIArray.LastElement(), aNameSpaceID);
 
@@ -244,9 +245,7 @@ nsNameSpaceManager::AddDisabledNameSpace(already_AddRefed<nsIAtom> aURI,
     return NS_ERROR_OUT_OF_MEMORY;
   }
 
-  NS_ASSERTION(aNameSpaceID - 1 == (int32_t) mURIArray.Length(),
-               "BAD! AddDisabledNameSpace not called in right order!");
-
+  MOZ_ASSERT(aNameSpaceID == (int32_t) mURIArray.Length());
   mURIArray.AppendElement(uri.forget());
   mDisabledURIToIDTable.Put(mURIArray.LastElement(), aNameSpaceID);
 
diff --git a/dom/base/nsNameSpaceManager.h b/dom/base/nsNameSpaceManager.h
index 6e9ed8e25204..d5c3a25fe819 100644
--- a/dom/base/nsNameSpaceManager.h
+++ b/dom/base/nsNameSpaceManager.h
@@ -42,9 +42,19 @@ public:
 
   virtual nsresult GetNameSpaceURI(int32_t aNameSpaceID, nsAString& aURI);
 
+  // Returns the atom for the namespace URI associated with the given ID. The
+  // ID must be within range and not be kNameSpaceID_None (i.e. zero);
   nsIAtom* NameSpaceURIAtom(int32_t aNameSpaceID) {
-    MOZ_ASSERT(aNameSpaceID > 0 && (int64_t) aNameSpaceID <= (int64_t) mURIArray.Length());
-    return mURIArray.ElementAt(aNameSpaceID - 1); // id is index + 1
+    MOZ_ASSERT(aNameSpaceID > 0);
+    return NameSpaceURIAtomForServo(aNameSpaceID);
+  }
+
+  // NB: This function should only be called by Servo code (and the above
+  // accessor), which uses the empty atom to represent kNameSpaceID_None.
+  nsIAtom* NameSpaceURIAtomForServo(int32_t aNameSpaceID) {
+    MOZ_ASSERT(aNameSpaceID >= 0);
+    MOZ_ASSERT((int64_t) aNameSpaceID < (int64_t) mURIArray.Length());
+    return mURIArray.ElementAt(aNameSpaceID);
   }
 
   int32_t GetNameSpaceID(const nsAString& aURI,
diff --git a/layout/style/ServoBindings.cpp b/layout/style/ServoBindings.cpp
index c888aed167e9..0c6e69df42fa 100644
--- a/layout/style/ServoBindings.cpp
+++ b/layout/style/ServoBindings.cpp
@@ -155,7 +155,7 @@ nsIAtom*
 Gecko_Namespace(RawGeckoElement* aElement)
 {
   int32_t id = aElement->NodeInfo()->NamespaceID();
-  return nsContentUtils::NameSpaceManager()->NameSpaceURIAtom(id);
+  return nsContentUtils::NameSpaceManager()->NameSpaceURIAtomForServo(id);
 }
 
 nsIAtom*