diff --git a/sherpa-onnx/csrc/sherpa-onnx-microphone-offline.cc b/sherpa-onnx/csrc/sherpa-onnx-microphone-offline.cc
index 7c5f9c8d..9a1f5825 100644
--- a/sherpa-onnx/csrc/sherpa-onnx-microphone-offline.cc
+++ b/sherpa-onnx/csrc/sherpa-onnx-microphone-offline.cc
@@ -8,6 +8,7 @@
 
 #include <algorithm>
 #include <cctype>  // std::tolower
+#include <mutex>   // NOLINT
 #include <thread>  // NOLINT
 
 #include "portaudio.h"  // NOLINT