diff --git a/include/spdlog/sinks/daily_file_sink.h b/include/spdlog/sinks/daily_file_sink.h
index 34797469..d4f22487 100644
--- a/include/spdlog/sinks/daily_file_sink.h
+++ b/include/spdlog/sinks/daily_file_sink.h
@@ -45,7 +45,7 @@ struct daily_filename_format_calculator
     // Create filename by formatting %Y_%m_%d.log
     static filename_t calc_filename (const filename_t &filename, const tm &now_tm)
     {
-        std::string fmt_filename = fmt::format("{{:{}}}", filename);
+        filename_t fmt_filename = fmt::format("{{:{}}}", filename);
         return fmt::format(fmt_filename, now_tm);
     }
 };