diff --git a/dom/base/nsNodeUtils.cpp b/dom/base/nsNodeUtils.cpp
index 862b5db14b..3670b54381 100644
--- a/dom/base/nsNodeUtils.cpp
+++ b/dom/base/nsNodeUtils.cpp
@@ -486,12 +486,12 @@ nsNodeUtils::CloneAndAdopt(nsINode *aNode, bool aClone, bool aDeep,
       CustomElementDefinition* definition = nullptr;
       RefPtr<nsIAtom> tagAtom = nodeInfo->NameAtom();
       if (nsContentUtils::IsCustomElementName(tagAtom)) {
+        elem->SetCustomElementData(new CustomElementData(tagAtom));
         definition =
           nsContentUtils::LookupCustomElementDefinition(nodeInfo->GetDocument(),
                                                         nodeInfo->LocalName(),
                                                         nodeInfo->NamespaceID());
         if (definition) {
-          elem->SetCustomElementData(new CustomElementData(tagAtom));
           nsContentUtils::EnqueueUpgradeReaction(elem, definition);
         }
       } else {
@@ -500,14 +500,14 @@ nsNodeUtils::CloneAndAdopt(nsINode *aNode, bool aClone, bool aDeep,
         nsAutoString extension;
         if (elem->GetAttr(kNameSpaceID_None, nsGkAtoms::is, extension) &&
             !extension.IsEmpty()) {
+          RefPtr<nsAtom> typeAtom = NS_Atomize(extension);
+          elem->SetCustomElementData(new CustomElementData(typeAtom));
           definition =
             nsContentUtils::LookupCustomElementDefinition(nodeInfo->GetDocument(),
                                                           nodeInfo->LocalName(),
                                                           nodeInfo->NamespaceID(),
                                                           &extension);
           if (definition) {
-            RefPtr<nsIAtom> typeAtom = NS_Atomize(extension);
-            elem->SetCustomElementData(new CustomElementData(typeAtom));
             nsContentUtils::EnqueueUpgradeReaction(elem, definition);
           }
         }
@@ -549,7 +549,7 @@ nsNodeUtils::CloneAndAdopt(nsINode *aNode, bool aClone, bool aDeep,
         // shadow-including inclusive descendants that is custom.
         Element* element = aNode->IsElement() ? aNode->AsElement() : nullptr;
         if (element) {
-          RefPtr<CustomElementData> data = element->GetCustomElementData();
+          CustomElementData* data = element->GetCustomElementData();
           if (data && data->mState == CustomElementData::State::eCustom) {
             LifecycleAdoptedCallbackArgs args = {
               oldDoc,
diff --git a/dom/tests/mochitest/webcomponents/test_custom_element_throw_on_dynamic_markup_insertion.html b/dom/tests/mochitest/webcomponents/test_custom_element_throw_on_dynamic_markup_insertion.html
new file mode 100644
index 0000000000..b5ef668605
--- /dev/null
+++ b/dom/tests/mochitest/webcomponents/test_custom_element_throw_on_dynamic_markup_insertion.html
@@ -0,0 +1,66 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=1378079
+-->
+<head>
+  <title>Test throw on dynamic markup insertion when creating element synchronously from parser</title>
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1378079">Bug 1378079</a>
+<div id="container"></div>
+
+<script>
+
+class DoDocumentOpenInCtor extends HTMLElement {
+  constructor() {
+    super();
+    document.open();
+  }
+};
+customElements.define('x-document-open-in-ctor', DoDocumentOpenInCtor);
+
+class DoDocumentWriteInCtor extends HTMLElement {
+  constructor() {
+    super();
+    document.write('<div>This should not be shown</div>');
+  }
+};
+customElements.define('x-document-write-in-ctor', DoDocumentWriteInCtor);
+
+class DoDocumentCloseInCtor extends HTMLElement {
+  constructor() {
+    super();
+    document.close();
+  }
+};
+customElements.define('x-document-close-in-ctor', DoDocumentCloseInCtor);
+
+window.errors = [];
+window.onerror = function(message, url, lineNumber, columnNumber, error) {
+  errors.push(error.name);
+  return true;
+}
+var expectedErrorCount = 0;
+
+document.write("<x-document-open-in-ctor></x-document-open-in-ctor>");
+expectedErrorCount++;
+is(window.errors.length, expectedErrorCount, "expectedErrorCount should be " + expectedErrorCount);
+is(window.errors[expectedErrorCount - 1], 'InvalidStateError', "Error name should be 'InvalidStateError'");
+
+document.write("<x-document-write-in-ctor></x-document-write-in-ctor>");
+expectedErrorCount++;
+is(window.errors.length, expectedErrorCount, "expectedErrorCount should be " + expectedErrorCount);
+is(window.errors[expectedErrorCount - 1], 'InvalidStateError', "Error name should be 'InvalidStateError'");
+
+document.write("<x-document-close-in-ctor></x-document-close-in-ctor>");
+expectedErrorCount++;
+is(window.errors.length, expectedErrorCount, "expectedErrorCount should be " + expectedErrorCount);
+is(window.errors[expectedErrorCount - 1], 'InvalidStateError', "Error name should be 'InvalidStateError'");
+
+</script>
+
+</body>
+</html>
