diff --git a/src/jupytext/cli.py b/src/jupytext/cli.py
index 4f2fb75d..376fd4f8 100644
--- a/src/jupytext/cli.py
+++ b/src/jupytext/cli.py
@@ -1189,7 +1189,7 @@ def pipe_notebook(
     command = shlex.split(command)
     if "{}" in command:
         if prefix is not None:
-            prefix = prefix + (" " if " " in prefix else "-")
+            prefix = prefix + "-"
         tmp_file_args = dict(
             mode="w+",
             encoding="utf8",