diff --git a/modules/system/activation/activation-script.nix b/modules/system/activation/activation-script.nix
index b87429008a5..744a35f54f4 100644
--- a/modules/system/activation/activation-script.nix
+++ b/modules/system/activation/activation-script.nix
@@ -79,7 +79,7 @@ let
       mkdir -m 0755 -p /var/run/console # for pam_console
 
       touch /var/run/utmp # must exist
-      chgrp ${config.ids.gids.utmp} /var/run/utmp
+      chgrp ${toString config.ids.gids.utmp} /var/run/utmp
       chmod 664 /var/run/utmp
 
       mkdir -m 0755 -p /var/run/nix/current-load # for distributed builds