diff --git a/nvflare/cli.py b/nvflare/cli.py
index 65d776c99f..4319ed9608 100644
--- a/nvflare/cli.py
+++ b/nvflare/cli.py
@@ -141,7 +141,7 @@ def parse_args(prog_name: str):
     cmd = args.__dict__.get("sub_command")
     sub_cmd_parser = sub_cmd_parsers.get(cmd)
     if argv:
-        msg = f"{prog_name} {cmd}: unrecognized arguments: {''.join(argv)}\n"
+        msg = f"{prog_name} {cmd}: unrecognized arguments: {' '.join(argv)}\n"
         print(f"\nerror: {msg}")
         sub_cmd_parser.print_help()
         _parser.exit(2, "\n")