diff --git a/rs.rkt b/rs.rkt
index a9af363..6bf4e32 100644
--- a/rs.rkt
+++ b/rs.rkt
@@ -37,7 +37,7 @@
 (define-type ReactionSystem (HashTable Symbol reaction))
 
 ;;; Returns the list of reaction names enabled on a given set.
-(: list-enabled (-> ReactionSystem (Setof Symbol) (Listof Symbol)))
+(: list-enabled (-> ReactionSystem (Setof Species) (Listof Symbol)))
 (define (list-enabled rs s)
   (for/list ([(name reaction) (in-hash rs)]
              #:when (enabled? reaction s))