diff --git a/include/spdlog/sinks/systemd_sink.h b/include/spdlog/sinks/systemd_sink.h
index da1c3e2c..ae8ace75 100644
--- a/include/spdlog/sinks/systemd_sink.h
+++ b/include/spdlog/sinks/systemd_sink.h
@@ -4,6 +4,7 @@
 #pragma once
 
 #include "spdlog/sinks/base_sink.h"
+#include "spdlog/details/null_mutex.h"
 #include "spdlog/details/synchronous_factory.h"
 
 #include <array>