diff --git a/networks.rkt b/networks.rkt
index 7310750..cdb99bf 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -87,7 +87,8 @@
  ;; Contracts
  (contract-out [state/c contract?]
                [update-function/c contract?]
-               [domain-mapping/c contract?]))
+               [domain-mapping/c contract?]
+               [tbn? contract?]))
 
 (module+ test
   (require rackunit))