diff --git a/etc/rc.d/init.d/functions b/etc/rc.d/init.d/functions

index 97f09d13..9b0add1c 100644

--- a/etc/rc.d/init.d/functions

+++ b/etc/rc.d/init.d/functions

@@ -84,8 +84,9 @@ if [ -z "${BOOTUP:-}" ]; then

         LOGLEVEL=1

     fi

-    # NOTE: /dev/ttyS* is serial console.

-    if tty | grep --quiet -e '/dev/ttyS'; then

+    # NOTE: /dev/ttyS* is serial console. "not a tty" is such as

+    # /dev/null associated when executed under systemd service units.

+    if LANG=C tty | grep --quiet -e '\(/dev/ttyS\|not a tty\)'; then

         BOOTUP=serial

         MOVE_TO_COL=

         SETCOLOR_SUCCESS=