diff --git a/networks.rkt b/networks.rkt
index 8139303..4080eab 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -955,5 +955,12 @@
 ;;; weights, as well as a threshold.
 (define (make-tbf/state pairs threshold)
   (tbf/state (make-immutable-hash pairs) threshold))
+
+(module+ test
+  (test-case "tbf/state"
+    (define f (make-tbf/state '((x1 . 1) (x2 . 1)) 1))
+    (check-equal? (tbf/state-w f) #hash((x1 . 1) (x2 . 1)))
+    (check-equal? (tbf/state-θ f) 1)))
+
 ;;; A TBN is a network form mapping TBFs to variables.
 (define tbn? (hash/c variable? tbf?))