diff --git a/ipc/glue/MessageLink.cpp b/ipc/glue/MessageLink.cpp
index 3adc4d548a59..bac1269b2625 100644
--- a/ipc/glue/MessageLink.cpp
+++ b/ipc/glue/MessageLink.cpp
@@ -73,6 +73,8 @@ class PortLink::PortObserverThunk : public NodeController::PortObserver {
 
 PortLink::PortLink(MessageChannel* aChan, ScopedPort aPort)
     : MessageLink(aChan), mNode(aPort.Controller()), mPort(aPort.Release()) {
+  MonitorAutoLock lock(*mChan->mMonitor);
+
   mObserver = new PortObserverThunk(mChan->mMonitor, this);
   mNode->SetPortObserver(mPort, mObserver);