diff --git a/include/spdlog/async_logger.h b/include/spdlog/async_logger.h
index 70ee3267..932d7fb3 100644
--- a/include/spdlog/async_logger.h
+++ b/include/spdlog/async_logger.h
@@ -61,7 +61,7 @@ protected:
     void sink_it_(details::log_msg &msg) override;
     void flush_() override;
 
-    void backend_log_(details::log_msg &incoming_log_msg);
+    void backend_log_(const details::log_msg &incoming_log_msg);
     void backend_flush_();
 
 private:
diff --git a/include/spdlog/details/async_logger_impl.h b/include/spdlog/details/async_logger_impl.h
index 3ad70a3b..55df4313 100644
--- a/include/spdlog/details/async_logger_impl.h
+++ b/include/spdlog/details/async_logger_impl.h
@@ -67,7 +67,7 @@ inline void spdlog::async_logger::flush_()
 //
 // backend functions - called from the thread pool to do the actual job
 //
-inline void spdlog::async_logger::backend_log_(details::log_msg &incoming_log_msg)
+inline void spdlog::async_logger::backend_log_(const details::log_msg &incoming_log_msg)
 {
     try
     {