diff --git a/pc/channel.cc b/pc/channel.cc
index 661f0e908c..69b2ca1676 100644
--- a/pc/channel.cc
+++ b/pc/channel.cc
@@ -30,6 +30,7 @@
 #include "rtc_base/logging.h"
 #include "rtc_base/network_route.h"
 #include "rtc_base/strings/string_builder.h"
+#include "rtc_base/synchronization/mutex.h"
 #include "rtc_base/synchronization/sequence_checker.h"
 #include "rtc_base/trace_event.h"
 
@@ -839,17 +840,14 @@ void BaseChannel::SignalSentPacket_n(const rtc::SentPacket& sent_packet) {
 void BaseChannel::SetNegotiatedHeaderExtensions_w(
     const RtpHeaderExtensions& extensions) {
   TRACE_EVENT0("webrtc", __func__);
-  RtpHeaderExtensions extensions_copy = extensions;
-  invoker_.AsyncInvoke<void>(
-      RTC_FROM_HERE, signaling_thread(),
-      [this, extensions_copy = std::move(extensions_copy)] {
-        RTC_DCHECK_RUN_ON(signaling_thread());
-        negotiated_header_extensions_ = std::move(extensions_copy);
-      });
+  RTC_DCHECK_RUN_ON(worker_thread());
+  webrtc::MutexLock lock(&negotiated_header_extensions_lock_);
+  negotiated_header_extensions_ = extensions;
 }
 
 RtpHeaderExtensions BaseChannel::GetNegotiatedRtpHeaderExtensions() const {
   RTC_DCHECK_RUN_ON(signaling_thread());
+  webrtc::MutexLock lock(&negotiated_header_extensions_lock_);
   return negotiated_header_extensions_;
 }
 
diff --git a/pc/channel.h b/pc/channel.h
index 5e2bd7e651..bbb95d7ea1 100644
--- a/pc/channel.h
+++ b/pc/channel.h
@@ -381,8 +381,13 @@ class BaseChannel : public ChannelInterface,
   // This object is not owned by the channel so it must outlive it.
   rtc::UniqueRandomIdGenerator* const ssrc_generator_;
 
+  // |negotiated_header_extensions_| is read on the signaling thread, but
+  // written on the worker thread while being sync-invoked from the signal
+  // thread in SdpOfferAnswerHandler::PushdownMediaDescription(). Hence the lock
+  // isn't strictly needed, but it's anyway placed here for future safeness.
+  mutable webrtc::Mutex negotiated_header_extensions_lock_;
   RtpHeaderExtensions negotiated_header_extensions_
-      RTC_GUARDED_BY(signaling_thread());
+      RTC_GUARDED_BY(negotiated_header_extensions_lock_);
 };
 
 // VoiceChannel is a specialization that adds support for early media, DTMF,