diff --git a/masque/pattern.py b/masque/pattern.py
index 7c5f8c3..4a401b6 100644
--- a/masque/pattern.py
+++ b/masque/pattern.py
@@ -1305,7 +1305,7 @@ class Pattern(PortList, AnnotatableImpl, Mirrorable):
 
         # If asked to inherit a name, check that all conditions are met
         if isinstance(thru, str):
-            if not len(map_in == 1):
+            if not len(map_in) == 1:
                 raise PatternError(f'Got {thru=} but have multiple map_in entries; don\'t know which one to use')
             if thru in map_out:
                 raise PatternError(f'Got {thru=} but tha port already exists in map_out')