diff --git a/gnu/services.scm b/gnu/services.scm
index f151bbaa9d..7de78105ff 100644
--- a/gnu/services.scm
+++ b/gnu/services.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2015, 2016, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2015, 2016, 2017, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2016 Chris Marusich <cmmarusich@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -649,7 +649,8 @@ as Wifi cards.")))
                 (extend append)
                 (description
                  "Register garbage-collector roots---i.e., store items that
-will not be reclaimed by the garbage collector.")))
+will not be reclaimed by the garbage collector.")
+                (default-value '())))
 
 
 ;;;