diff --git a/netwerk/streamconv/converters/nsUnknownDecoder.cpp b/netwerk/streamconv/converters/nsUnknownDecoder.cpp
index beb101984334..2919e816b9ce 100644
--- a/netwerk/streamconv/converters/nsUnknownDecoder.cpp
+++ b/netwerk/streamconv/converters/nsUnknownDecoder.cpp
@@ -412,6 +412,8 @@ void nsUnknownDecoder::DetermineContentType(nsIRequest* aRequest) {
   if (channel) {
     nsCOMPtr<nsILoadInfo> loadInfo = channel->LoadInfo();
     if (loadInfo->GetSkipContentSniffing()) {
+      MutexAutoLock lock(mMutex);
+      mContentType = UNKNOWN_CONTENT_TYPE;
       return;
     }
   }