diff --git a/core/classes/tuple/tuple.factor b/core/classes/tuple/tuple.factor
index df59f34ff4..6056d200be 100755
--- a/core/classes/tuple/tuple.factor
+++ b/core/classes/tuple/tuple.factor
@@ -103,7 +103,7 @@ ERROR: bad-superclass class ;
 
 : superclass-size ( class -- n )
     superclasses but-last-slice
-    [ slot-names length ] map sum ;
+    [ slot-names length ] sigma ;
 
 : generate-tuple-slots ( class slots -- slot-specs )
     over superclass-size 2 + simple-slots ;