diff --git a/install/alternc.install b/install/alternc.install
index 0f1e84e8..692460d5 100755
--- a/install/alternc.install
+++ b/install/alternc.install
@@ -654,7 +654,7 @@ if [ "$(lsb_release -s -c)" == 'stretch' ] ; then
     # Without adding '-u opendkim' after the service file is generated, opendkim
     # will run as root, which we do not want.
     if [ "$(grep -c 'u opendkim' /etc/systemd/system/opendkim.service.d/override.conf)" == 0 ] ; then
-        sed 's/inet:8891@127.0.0.1/& -u opendkim/' /etc/systemd/system/opendkim.service.d/override.conf
+        sed -i -e 's/inet:8891@127.0.0.1/& -u opendkim/' /etc/systemd/system/opendkim.service.d/override.conf
     fi
     systemctl daemon-reload
 fi