diff --git a/CARET_trace/include/caret_trace/tracing_controller.hpp b/CARET_trace/include/caret_trace/tracing_controller.hpp
index 345f68ba..ffe67910 100644
--- a/CARET_trace/include/caret_trace/tracing_controller.hpp
+++ b/CARET_trace/include/caret_trace/tracing_controller.hpp
@@ -186,6 +186,7 @@ class TracingController
   void info(std::string message) const;
 
   std::shared_timed_mutex mutex_;
+  std::shared_timed_mutex nested_mutex_;
   std::string to_node_name(const void * callback);
   std::string to_topic_name(const void * callback);
 
diff --git a/CARET_trace/src/tracing_controller.cpp b/CARET_trace/src/tracing_controller.cpp
index f5396ea6..9ad7ef94 100644
--- a/CARET_trace/src/tracing_controller.cpp
+++ b/CARET_trace/src/tracing_controller.cpp
@@ -1009,7 +1009,7 @@ void TracingController::add_client_handle(const void * client_handle, const void
 
 void TracingController::add_allowed_messages(const void * message, bool is_allowed)
 {
-  // Be sure to call with "mutex_" locked.
+  std::lock_guard<std::shared_timed_mutex> lock(nested_mutex_);
   static const int max_sz = 256;
   if (allowed_messages_.size() > max_sz) {
     allowed_messages_.clear();