diff --git a/talk/session/media/channelmanager.cc b/talk/session/media/channelmanager.cc
index 45e7e4764a..f40396d928 100644
--- a/talk/session/media/channelmanager.cc
+++ b/talk/session/media/channelmanager.cc
@@ -218,6 +218,12 @@ bool ChannelManager::Init() {
 
   ASSERT(worker_thread_ != NULL);
   if (worker_thread_) {
+    if (worker_thread_ != rtc::Thread::Current()) {
+      // Do not allow invoking calls to other threads on the worker thread.
+      worker_thread_->Invoke<bool>(rtc::Bind(
+          &rtc::Thread::SetAllowBlockingCalls, worker_thread_, false));
+    }
+
     if (media_engine_->Init(worker_thread_)) {
       initialized_ = true;
 
diff --git a/webrtc/base/thread.h b/webrtc/base/thread.h
index 34ec45e3b9..8276f4e8ec 100644
--- a/webrtc/base/thread.h
+++ b/webrtc/base/thread.h
@@ -210,7 +210,7 @@ class Thread : public MessageQueue {
   bool RunningForTest() { return running(); }
 
   // Sets the per-thread allow-blocking-calls flag and returns the previous
-  // value.
+  // value. Must be called on this thread.
   bool SetAllowBlockingCalls(bool allow);
 
  protected: