diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index af04f3b31831..69311a72a2cd 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -1253,37 +1253,51 @@ CustomElementRegistry::CallGetCustomInterface(Element* aElement,
 {
   MOZ_ASSERT(aElement);
 
-  if (nsContentUtils::IsChromeDoc(aElement->OwnerDoc())) {
-    CustomElementDefinition* definition = aElement->GetCustomElementDefinition();
-    if (definition && definition->mCallbacks &&
-        definition->mCallbacks->mGetCustomInterfaceCallback.WasPassed() &&
-        definition->mLocalName == aElement->NodeInfo()->NameAtom()) {
-
-      LifecycleGetCustomInterfaceCallback* func =
-        definition->mCallbacks->mGetCustomInterfaceCallback.Value();
-      JS::Rooted<JSObject*> customInterface(RootingCx());
-
-      nsCOMPtr<nsIJSID> iid = nsJSID::NewID(aIID);
-      func->Call(aElement, iid, &customInterface);
-      JS::Rooted<JSObject*> funcGlobal(RootingCx(), func->CallbackGlobalOrNull());
-      if (customInterface && funcGlobal) {
-        AutoJSAPI jsapi;
-        if (jsapi.Init(funcGlobal)) {
-          nsIXPConnect *xpConnect = nsContentUtils::XPConnect();
-          JSContext* cx = jsapi.cx();
-
-          nsCOMPtr<nsISupports> wrapper;
-          nsresult rv = xpConnect->WrapJSAggregatedToNative(aElement, cx, customInterface,
-                                                            aIID, getter_AddRefs(wrapper));
-          if (NS_SUCCEEDED(rv)) {
-            return wrapper.forget();
-          }
-        }
-      }
-    }
+  if (!nsContentUtils::IsChromeDoc(aElement->OwnerDoc())) {
+    return nullptr;
   }
 
-  return nullptr;
+  // Try to get our GetCustomInterfaceCallback callback.
+  CustomElementDefinition* definition = aElement->GetCustomElementDefinition();
+  if (!definition || !definition->mCallbacks ||
+      !definition->mCallbacks->mGetCustomInterfaceCallback.WasPassed() ||
+      (definition->mLocalName != aElement->NodeInfo()->NameAtom())) {
+    return nullptr;
+  }
+  LifecycleGetCustomInterfaceCallback* func =
+    definition->mCallbacks->mGetCustomInterfaceCallback.Value();
+
+  // Initialize a AutoJSAPI to enter the compartment of the callback.
+  AutoJSAPI jsapi;
+  JS::RootedObject funcGlobal(RootingCx(), func->CallbackGlobalOrNull());
+  if (!funcGlobal || !jsapi.Init(funcGlobal)) {
+    return nullptr;
+  }
+
+  // Grab our JSContext.
+  JSContext* cx = jsapi.cx();
+
+  // Convert our IID to a JSValue to call our callback.
+  JS::RootedValue jsiid(cx);
+  if (!xpc::ID2JSValue(cx, aIID, &jsiid)) {
+    return nullptr;
+  }
+
+  JS::RootedObject customInterface(cx);
+  func->Call(aElement, jsiid, &customInterface);
+  if (!customInterface) {
+    return nullptr;
+  }
+
+  // Wrap our JSObject into a nsISupports through XPConnect
+  nsCOMPtr<nsISupports> wrapper;
+  nsresult rv = nsContentUtils::XPConnect()->WrapJSAggregatedToNative(
+    aElement, cx, customInterface, aIID, getter_AddRefs(wrapper));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return nullptr;
+  }
+
+  return wrapper.forget();
 }
 
 //-----------------------------------------------------
diff --git a/dom/webidl/WebComponents.webidl b/dom/webidl/WebComponents.webidl
index 23343fd8d932..cc254c41d14c 100644
--- a/dom/webidl/WebComponents.webidl
+++ b/dom/webidl/WebComponents.webidl
@@ -10,8 +10,6 @@
  * liability, trademark and document use rules apply.
  */
 
-interface IID;
-
 callback LifecycleConnectedCallback = void();
 callback LifecycleDisconnectedCallback = void();
 callback LifecycleAdoptedCallback = void(Document? oldDocument,
@@ -20,7 +18,7 @@ callback LifecycleAttributeChangedCallback = void(DOMString attrName,
                                                   DOMString? oldValue,
                                                   DOMString? newValue,
                                                   DOMString? namespaceURI);
-callback LifecycleGetCustomInterfaceCallback = object?(IID iid);
+callback LifecycleGetCustomInterfaceCallback = object?(any iid);
 
 dictionary LifecycleCallbacks {
   LifecycleConnectedCallback connectedCallback;