diff --git a/guix/derivations.scm b/guix/derivations.scm
index b13664ce05..151bff7215 100644
--- a/guix/derivations.scm
+++ b/guix/derivations.scm
@@ -290,8 +290,8 @@ (define (env-vars-with-empty-outputs)
       (fold-right (lambda (output-name env-vars)
                     (if (assoc output-name env-vars)
                         env-vars
-                        (alist-cons output-name "" env-vars)))
-                  '()
+                        (append env-vars `((,output-name . "")))))
+                  e
                   outputs)))
 
   (let* ((outputs    (map (lambda (name)