diff --git a/dom/ipc/ProcessHangMonitor.cpp b/dom/ipc/ProcessHangMonitor.cpp
index d46a1f5d5e..406d0bf0aa 100644
--- a/dom/ipc/ProcessHangMonitor.cpp
+++ b/dom/ipc/ProcessHangMonitor.cpp
@@ -323,6 +323,8 @@ HangMonitorChild::Shutdown()
 {
   MOZ_RELEASE_ASSERT(NS_IsMainThread());
 
+  BackgroundHangMonitor::UnregisterAnnotator(*this);
+
   MonitorAutoLock lock(mMonitor);
   while (!mShutdownDone) {
     mMonitor.Wait();