diff --git a/csys/csys.lisp b/csys/csys.lisp
index 634b15b..06a91b7 100644
--- a/csys/csys.lisp
+++ b/csys/csys.lisp
@@ -34,8 +34,7 @@
   (setf (gethash key (actions *environment*)) fn))
 
 (defun add-sensor (key sn)
-  (let ((sns (sensors *environment*)))
-    (setf (gethash key sns) (adjoin sn (gethash key sns)))))
+  (pushnew sn (gethash key (sensors *environment*))))
 
 ;;;; neurons (= behavior factories) and synapses (connection factories)