diff --git a/base/threaded_container.cpp b/base/threaded_container.cpp
index 0f3b371312..825bce8eb4 100644
--- a/base/threaded_container.cpp
+++ b/base/threaded_container.cpp
@@ -7,6 +7,7 @@ ThreadedContainer::ThreadedContainer()
 
 void ThreadedContainer::Cancel()
 {
+  threads::ConditionGuard g(m_Cond);
   m_IsCancelled = true;
   m_Cond.Signal(true);
 }