diff --git a/f8/app.cc b/f8/app.cc
index 0df1ab5..a5c8790 100644
--- a/f8/app.cc
+++ b/f8/app.cc
@@ -196,10 +196,11 @@ namespace f8
         std::string proj_log_root_dir = a8::Format
             (PROJ_LOG_ROOT_FMT, {user_app_->GetPkgName()});
         std::string log_file_name = a8::Format
-            (PROJ_LOG_ROOT_FMT, {user_app_->GetPkgName() + "/" + filename_fmt});
+            (PROJ_LOG_ROOT_FMT, {user_app_->GetPkgName()}) + "/" + filename_fmt;
 
         a8::MkDir(proj_root_dir);
         a8::MkDir(proj_log_root_dir);
+        a8::XPrintf("log_file_name:%s\n", {log_file_name});
         f8::UdpLog::Instance()->SetLogFileName(log_file_name);
         f8::UdpLog::Instance()->Init();
         f8::UdpLog::Instance()->Info("proj_root_dir:%s", {proj_root_dir});