diff --git a/networks.rkt b/networks.rkt
index cda1dc7..56eb9f1 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -76,7 +76,7 @@
 (define-syntax-rule (st mappings) (make-state mappings))
 
 ;;; A version of make-hash restricted to creating networks.
-(define (make-network-from-functions funcs) (make-hash funcs))
+(define (make-network-from-functions funcs) (make-immutable-hash funcs))
 
 
 ;;; =================================
@@ -98,13 +98,13 @@
 
 ;;; Build a network from a network form.
 (define (network-form->network bnf)
-  (make-hash
+  (make-immutable-hash
    (hash-map bnf (λ (x form)
                    (cons x (update-function-form->update-function form))))))
 
 ;;; Build a network from a list of pairs of forms of update functions.
 (define (make-network-from-forms forms)
-  (network-form->network (make-hash forms)))
+  (network-form->network (make-immutable-hash forms)))
 
 ;;; A shortcut for make-network-from-forms.
 (define-syntax-rule (nn forms) (make-network-from-forms forms))