--- a/dom/xhr/XMLHttpRequestMainThread.cpp
+++ b/dom/xhr/XMLHttpRequestMainThread.cpp
@@ -173,16 +173,17 @@ static void AddLoadFlags(nsIRequest *req
 //
 /////////////////////////////////////////////
 
 bool
 XMLHttpRequestMainThread::sDontWarnAboutSyncXHR = false;
 
 XMLHttpRequestMainThread::XMLHttpRequestMainThread()
   : mResponseBodyDecodedPos(0),
+    mResponseCharset(nullptr),
     mResponseType(XMLHttpRequestResponseType::_empty),
     mRequestObserver(nullptr),
     mState(State::unsent),
     mFlagSynchronous(false), mFlagAborted(false), mFlagParseBody(false),
     mFlagSyncLooping(false), mFlagBackgroundRequest(false),
     mFlagHadUploadListenersOnSend(false), mFlagACwithCredentials(false),
     mFlagTimedOut(false), mFlagDeleted(false), mFlagSend(false),
     mUploadTransferred(0), mUploadTotal(0), mUploadComplete(true),