diff --git a/functions.rkt b/functions.rkt
index 4ab12c2..33084e7 100644
--- a/functions.rkt
+++ b/functions.rkt
@@ -12,7 +12,7 @@
 (provide
  ;; Structures
  (contract-out
-  [struct tbf ((weights number?) (threshold (vectorof number?)))])
+  [struct tbf ((weights (vectorof number?)) (threshold number?))])
  ;; Functions
  (contract-out
   [tabulate (-> procedure-fixed-arity? (listof generic-set?) (listof list?))]