diff --git a/dom/xml/nsXMLPrettyPrinter.cpp b/dom/xml/nsXMLPrettyPrinter.cpp
index 7f1f2b07a8..3573523186 100644
--- a/dom/xml/nsXMLPrettyPrinter.cpp
+++ b/dom/xml/nsXMLPrettyPrinter.cpp
@@ -256,6 +256,8 @@ nsXMLPrettyPrinter::ContentRemoved(nsIDocument* aDocument,
 void
 nsXMLPrettyPrinter::NodeWillBeDestroyed(const nsINode* aNode)
 {
+    MOZ_DIAGNOSTIC_ASSERT(mDocument == aNode);
+    mDocument->RemoveMutationObserver(this);
     mDocument = nullptr;
     NS_RELEASE_THIS();
 }