diff --git a/data/tools/unit_tree/html_output.py b/data/tools/unit_tree/html_output.py
index 7a5068d4ab80..dc165779a54a 100644
--- a/data/tools/unit_tree/html_output.py
+++ b/data/tools/unit_tree/html_output.py
@@ -330,7 +330,7 @@ def group_name(self, group):
                 name = name[name.rfind("=") + 1:]
             else:
                 name = "missing"
-                error_message("Warning: %s has no faction name\n", group[1])
+                error_message("Warning: %s has no faction name\n" % group[1])
         else:
             name = "factionless"
         return name