diff --git a/core/sets/sets.factor b/core/sets/sets.factor
index 06f6e04655..5dcee675f9 100644
--- a/core/sets/sets.factor
+++ b/core/sets/sets.factor
@@ -109,7 +109,7 @@ M: sequence null?
 M: sequence cardinality
     length ;
 
-: combine ( sets -- set )
+: combine ( sets -- set/f )
     [ f ]
     [ [ [ members ] map concat ] [ first ] bi set-like ]
     if-empty ;