diff --git a/core/classes/union/union-tests.factor b/core/classes/union/union-tests.factor
index 164c51f371..6f927441a4 100644
--- a/core/classes/union/union-tests.factor
+++ b/core/classes/union/union-tests.factor
@@ -132,3 +132,7 @@ PREDICATE: numba-ova-10 < union{ float integer }
 [ t ] [ 100.0 numba-ova-10? ] unit-test
 [ f ] [ 5 numba-ova-10? ] unit-test
 [ f ] [ 5.75 numba-ova-10? ] unit-test
+
+! Issue #420 lol
+[ "IN: issue-420 UNION: omg omg ;" eval( -- ) ]
+[ error>> cannot-reference-self? ] must-fail-with
diff --git a/core/classes/union/union.factor b/core/classes/union/union.factor
index 13563a1178..2a0afef3a1 100644
--- a/core/classes/union/union.factor
+++ b/core/classes/union/union.factor
@@ -53,10 +53,15 @@ M: union-class update-class define-union-predicate ;
 : (define-union-class) ( class members -- )
     f swap f union-class make-class-props (define-class) ;
 
+ERROR: cannot-reference-self class members ;
+
+: check-self-reference ( class members -- class members )
+    2dup member-eq? [ cannot-reference-self ] when ;
+
 PRIVATE>
 
 : define-union-class ( class members -- )
-    [ (define-union-class) ]
+    [ check-self-reference (define-union-class) ]
     [ drop changed-conditionally ]
     [ drop update-classes ]
     2tri ;