diff --git a/dom/base/nsTreeSanitizer.cpp b/dom/base/nsTreeSanitizer.cpp
index 8ee5120bf6d..c13ff63b026 100644
--- a/dom/base/nsTreeSanitizer.cpp
+++ b/dom/base/nsTreeSanitizer.cpp
@@ -1450,6 +1450,11 @@ void nsTreeSanitizer::SanitizeChildren(nsINode* aRoot) {
       NS_ASSERTION(ns == kNameSpaceID_XHTML || ns == kNameSpaceID_SVG ||
                        ns == kNameSpaceID_MathML,
                    "Should have only HTML, MathML or SVG here!");
+      if (elt->HasCustomElementData()) {
+        MOZ_ASSERT(elt->GetCustomElementData()->GetIs(elt),
+                   "CustomElementData without an |is| attribute?");
+        elt->ClearCustomElementData();
+      }
       AllowedAttributes allowed;
       if (ns == kNameSpaceID_XHTML) {
         allowed.mNames = sAttributesHTML;