diff --git a/sphinx/writers/manpage.py b/sphinx/writers/manpage.py
index 5421fb76314..124be7fdce5 100644
--- a/sphinx/writers/manpage.py
+++ b/sphinx/writers/manpage.py
@@ -115,7 +115,7 @@ def header(self) -> str:
                 " \"%(date)s\" \"%(version)s\" \"%(manual_group)s\"\n"
                 ".SH NAME\n")
         if self._docinfo['subtitle']:
-                "%(title)s \\- %(subtitle)s\n")
+                tmpl += "%(title)s \\- %(subtitle)s\n"
         return tmpl % self._docinfo
 
     def visit_start_of_file(self, node: Element) -> None: