diff --git a/masque/builder/tools.py b/masque/builder/tools.py
index b5ca9ca..8cd3a8f 100644
--- a/masque/builder/tools.py
+++ b/masque/builder/tools.py
@@ -1541,7 +1541,7 @@ class AutoTool(Tool):
             ) -> ILibrary:
 
         tree, pat = Library.mktree(SINGLE_USE_PREFIX + 'traceL')
-        pat.add_port_pair(names=(port_names[0], port_names[1]))
+        pat.add_port_pair(names=port_names)
 
         for step in batch:
             assert step.tool == self