diff --git a/core/sets/sets.factor b/core/sets/sets.factor
index ae15908e4c..a73515ca91 100644
--- a/core/sets/sets.factor
+++ b/core/sets/sets.factor
@@ -61,7 +61,7 @@ M: set subset?
     sequence/tester all? ;
 
 M: set set=
-    2dup subset? [ swap subset? ] [ 2drop f ] if ;
+    2dup [ cardinality ] bi@ = [ subset? ] [ 2drop f ] if ;
 
 M: set fast-set ;