diff --git a/core/classes/maybe/maybe.factor b/core/classes/maybe/maybe.factor
index 1c1d24a102..9c89e368f7 100644
--- a/core/classes/maybe/maybe.factor
+++ b/core/classes/maybe/maybe.factor
@@ -1,6 +1,6 @@
 ! Copyright (C) 2011 Doug Coleman.
 ! See http://factorcode.org/license.txt for BSD license.
-USING: accessors classes classes.algebra
+USING: accessors arrays classes classes.algebra
 classes.algebra.private classes.private classes.union.private
 kernel words ;
 IN: classes.maybe
@@ -41,3 +41,6 @@ M: maybe class-name
 
 M: maybe predicate-def
     class>> predicate-def [ [ t ] if* ] curry ;
+
+M: maybe classes-contained-by
+    class>> 1array ;
diff --git a/core/classes/union/union.factor b/core/classes/union/union.factor
index c2e6af41ff..a9c833f285 100644
--- a/core/classes/union/union.factor
+++ b/core/classes/union/union.factor
@@ -56,12 +56,17 @@ M: union-class update-class define-union-predicate ;
 
 ERROR: cannot-reference-self class members ;
 
-: union-members ( union -- members )
+GENERIC: classes-contained-by ( obj -- members )
+
+M: union-class classes-contained-by ( union -- members )
+    "members" word-prop [ f ] when-empty ;
+
+M: object classes-contained-by
     "members" word-prop [ f ] when-empty ;
 
 : check-self-reference ( class members -- class members )
     2dup [
-        dup dup [ union-members ] map concat sift append
+        dup dup [ classes-contained-by ] map concat sift append
         2dup set= [ 2drop f ] [ nip ] if
     ] follow concat
     member-eq? [ cannot-reference-self ] when ;