diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 41b97a56189b..b2bbba8e5fee 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -1177,12 +1177,14 @@ Element::SetAttribute(const nsAString& aName,
                       const nsAString& aValue,
                       ErrorResult& aError)
 {
-  aError = nsContentUtils::CheckQName(aName, false);
-  if (aError.Failed()) {
-    return;
-  }
   const nsAttrName* name = InternalGetExistingAttrNameFromQName(aName);
+
   if (!name) {
+    aError = nsContentUtils::CheckQName(aName, false);
+    if (aError.Failed()) {
+      return;
+    }
+
     nsCOMPtr<nsIAtom> nameAtom;
     if (IsHTMLElement() && IsInHTMLDocument()) {
       nsAutoString lower;