diff --git a/core/classes/union/union-tests.factor b/core/classes/union/union-tests.factor
index 6f927441a4..aeadd687c2 100644
--- a/core/classes/union/union-tests.factor
+++ b/core/classes/union/union-tests.factor
@@ -136,3 +136,10 @@ PREDICATE: numba-ova-10 < union{ float integer }
 ! Issue #420 lol
 [ "IN: issue-420 UNION: omg omg ;" eval( -- ) ]
 [ error>> cannot-reference-self? ] must-fail-with
+
+IN: issue-420
+UNION: a ;
+UNION: b a ;
+
+[ "IN: issue-420 UNION: a b ;" eval( -- ) ]
+[ error>> cannot-reference-self? ] must-fail-with
diff --git a/core/classes/union/union.factor b/core/classes/union/union.factor
index 2a0afef3a1..c2e6af41ff 100644
--- a/core/classes/union/union.factor
+++ b/core/classes/union/union.factor
@@ -4,6 +4,7 @@ USING: accessors assocs classes classes.algebra
 classes.algebra.private classes.builtin classes.private
 combinators definitions kernel kernel.private math math.private
 quotations sequences words ;
+FROM: sets => set= ;
 IN: classes.union
 
 PREDICATE: union-class < class
@@ -55,8 +56,15 @@ M: union-class update-class define-union-predicate ;
 
 ERROR: cannot-reference-self class members ;
 
+: union-members ( union -- members )
+    "members" word-prop [ f ] when-empty ;
+
 : check-self-reference ( class members -- class members )
-    2dup member-eq? [ cannot-reference-self ] when ;
+    2dup [
+        dup dup [ union-members ] map concat sift append
+        2dup set= [ 2drop f ] [ nip ] if
+    ] follow concat
+    member-eq? [ cannot-reference-self ] when ;
 
 PRIVATE>