diff --git a/ultramodern/src/save.cpp b/ultramodern/src/save.cpp
index 5b1b69e..74f6927 100644
--- a/ultramodern/src/save.cpp
+++ b/ultramodern/src/save.cpp
@@ -1,4 +1,5 @@
 #include <filesystem>
+#include <mutex>
 #include <thread>
 #include <vector>
 #include <ultramodern/files.hpp>