diff --git a/test/fake_encoder.cc b/test/fake_encoder.cc
index bc23c32dd2..9e1e1fe3c5 100644
--- a/test/fake_encoder.cc
+++ b/test/fake_encoder.cc
@@ -147,6 +147,7 @@ FakeEncoder::FrameInfo FakeEncoder::NextFrame(
     }
   }
 
+  rtc::CritScope cs(&crit_sect_);
   for (uint8_t i = 0; i < num_simulcast_streams; ++i) {
     if (target_bitrate.GetBitrate(i, 0) > 0) {
       int temporal_id = last_frame_info_.layers.size() > i
diff --git a/test/fake_encoder.h b/test/fake_encoder.h
index 67acebec69..9eddb0f1d3 100644
--- a/test/fake_encoder.h
+++ b/test/fake_encoder.h
@@ -71,7 +71,7 @@ class FakeEncoder : public VideoEncoder {
                       SimulcastStream simulcast_streams[kMaxSimulcastStreams],
                       int framerate);
 
-  FrameInfo last_frame_info_;
+  FrameInfo last_frame_info_ RTC_GUARDED_BY(crit_sect_);
   Clock* const clock_;
 
   VideoCodec config_ RTC_GUARDED_BY(crit_sect_);