diff --git a/networks.rkt b/networks.rkt
index 500cdc5..cda1dc7 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -70,7 +70,7 @@
 
 ;;; A version of make-hash restricted to creating network states (see
 ;;; contract).
-(define (make-state mappings) (make-hash mappings))
+(define (make-state mappings) (make-immutable-hash mappings))
 
 ;;; A shortcut for make-state.
 (define-syntax-rule (st mappings) (make-state mappings))