diff --git a/media/base/video_broadcaster.cc b/media/base/video_broadcaster.cc
index e6a91368fc..3c20eca963 100644
--- a/media/base/video_broadcaster.cc
+++ b/media/base/video_broadcaster.cc
@@ -94,6 +94,7 @@ void VideoBroadcaster::OnFrame(const webrtc::VideoFrame& frame) {
 }
 
 void VideoBroadcaster::OnDiscardedFrame() {
+  webrtc::MutexLock lock(&sinks_and_wants_lock_);
   for (auto& sink_pair : sink_pairs()) {
     sink_pair.sink->OnDiscardedFrame();
   }