--- a/dom/xhr/XMLHttpRequestMainThread.cpp
+++ b/dom/xhr/XMLHttpRequestMainThread.cpp
@@ -568,16 +568,21 @@ XMLHttpRequestMainThread::AppendToRespon
     AsBytes(MakeSpan(aSrcBuffer, aSrcBufferLen)),
     MakeSpan(helper.EndOfExistingData(), destBufferLen.value()),
     aLast);
   MOZ_ASSERT(result == kInputEmpty);
   MOZ_ASSERT(read == aSrcBufferLen);
   MOZ_ASSERT(written <= destBufferLen.value());
   Unused << hadErrors;
   helper.AddLength(written);
+
+  if (aLast) {
+    mDecoder = nullptr;
+  }
+
   return NS_OK;
 }
 
 NS_IMETHODIMP
 XMLHttpRequestMainThread::GetResponseText(nsAString& aResponseText)
 {
   ErrorResult rv;
   DOMString str;