diff --git a/os2.rkt b/os2.rkt
index baaace3..abfaffe 100644
--- a/os2.rkt
+++ b/os2.rkt
@@ -265,9 +265,9 @@
 (define state/c (not/c transition?))
 (define action/c action?)
 (define action-tree/c (flat-rec-contract action-tree/c
-					 (or action/c
-					     null?
-					     (cons/c action-tree/c action-tree/c))))
+					 (or/c action/c
+					       null?
+					       (cons/c action-tree/c action-tree/c))))
 
 (define make-add-role add-role) ;; no special treatment required at present
 (define (make-delete-role pre-eid [reason #f]) (delete-role pre-eid reason))