diff --git a/include/logger.hpp b/include/logger.hpp
index 23aceed..8009c23 100644
--- a/include/logger.hpp
+++ b/include/logger.hpp
@@ -62,7 +62,7 @@ public:
 
 private:
     LogLevel level_;
-    std::mutex mutex_;
+    mutable std::mutex mutex_;
 
     /// @brief Convert a LogLevel to its string representation.
     static std::string levelToString(LogLevel level);