--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -10549,16 +10549,17 @@ public:
 
   explicit UnblockParsingPromiseHandler(nsIDocument* aDocument, Promise* aPromise)
     : mPromise(aPromise)
   {
     nsCOMPtr<nsIParser> parser = aDocument->CreatorParserOrNull();
     if (parser) {
       parser->BlockParser();
       mParser = do_GetWeakReference(parser);
+      mDocument = aDocument;
     }
   }
 
   void
   ResolvedCallback(JSContext* aCx, JS::Handle<JS::Value> aValue) override
   {
     MaybeUnblockParser();
 
@@ -10578,27 +10579,33 @@ protected:
   {
     MaybeUnblockParser();
   }
 
 private:
   void MaybeUnblockParser() {
     nsCOMPtr<nsIParser> parser = do_QueryReferent(mParser);
     if (parser) {
-      parser->UnblockParser();
-      parser->ContinueInterruptedParsingAsync();
-      mParser = nullptr;
-    }
+      MOZ_ASSERT(mDocument);
+      nsCOMPtr<nsIParser> docParser = mDocument->CreatorParserOrNull();
+      if (parser == docParser) {
+        parser->UnblockParser();
+        parser->ContinueInterruptedParsingAsync();
+      }
+    }
+    mParser = nullptr;
+    mDocument = nullptr;
   }
 
   nsWeakPtr mParser;
   RefPtr<Promise> mPromise;
+  RefPtr<nsIDocument> mDocument;
 };
 
-NS_IMPL_CYCLE_COLLECTION(UnblockParsingPromiseHandler, mPromise)
+NS_IMPL_CYCLE_COLLECTION(UnblockParsingPromiseHandler, mDocument, mPromise)
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(UnblockParsingPromiseHandler)
   NS_INTERFACE_MAP_ENTRY(nsISupports)
 NS_INTERFACE_MAP_END
 
 NS_IMPL_CYCLE_COLLECTING_ADDREF(UnblockParsingPromiseHandler)
 NS_IMPL_CYCLE_COLLECTING_RELEASE(UnblockParsingPromiseHandler)
 

--- a/parser/htmlparser/nsParser.cpp
+++ b/parser/htmlparser/nsParser.cpp
@@ -677,17 +677,20 @@ nsParser::UnblockParser()
   if (MOZ_LIKELY(mBlocked > 0)) {
     mBlocked--;
   }
 }
 
 NS_IMETHODIMP_(void)
 nsParser::ContinueInterruptedParsingAsync()
 {
-  mSink->ContinueInterruptedParsingAsync();
+  MOZ_ASSERT(mSink);
+  if (MOZ_LIKELY(mSink)) {
+    mSink->ContinueInterruptedParsingAsync();
+  }
 }
 
 /**
  * Call this to query whether the parser is enabled or not.
  */
 NS_IMETHODIMP_(bool)
 nsParser::IsParserEnabled()
 {