diff --git a/pc/peer_connection.h b/pc/peer_connection.h
index 2d07893..07419bf 100644
--- a/pc/peer_connection.h
+++ b/pc/peer_connection.h
@@ -1125,10 +1125,14 @@
       RTC_GUARDED_BY(signaling_thread());
 
   // These lists store sender info seen in local/remote descriptions.
-  std::vector<RtpSenderInfo> remote_audio_sender_infos_;
-  std::vector<RtpSenderInfo> remote_video_sender_infos_;
-  std::vector<RtpSenderInfo> local_audio_sender_infos_;
-  std::vector<RtpSenderInfo> local_video_sender_infos_;
+  std::vector<RtpSenderInfo> remote_audio_sender_infos_
+      RTC_GUARDED_BY(signaling_thread());
+  std::vector<RtpSenderInfo> remote_video_sender_infos_
+      RTC_GUARDED_BY(signaling_thread());
+  std::vector<RtpSenderInfo> local_audio_sender_infos_
+      RTC_GUARDED_BY(signaling_thread());
+  std::vector<RtpSenderInfo> local_video_sender_infos_
+      RTC_GUARDED_BY(signaling_thread());
 
   SctpSidAllocator sid_allocator_;
   // label -> DataChannel