diff --git a/scripts/upgrade b/scripts/upgrade
index a3a486c..5de8660 100644
--- a/scripts/upgrade
+++ b/scripts/upgrade
@@ -55,32 +55,32 @@ enable_registration_captcha=$(ynh_app_setting_get --app=$app --key=enable_regist
 
 
 # Define $server_name if not already defined
-if [ -z $server_name ]; then
+if [ -z ${server_name:-} ]; then
    server_name=$domain
    ynh_app_setting_set --app=$app --key=server_name --value=$domain
 fi
 
 # Define $disable_federation if not already defined
-if [ -z $enable_registration_captcha ]; then
+if [ -z ${enable_registration_captcha:-} ]; then
    enable_registration_captcha="false"
    ynh_app_setting_set --app=$app --key=enable_registration_captcha --value=$enable_registration_captcha
 fi
 
 # Define $guests_disabled if not already defined
-if [ -z $guests_disabled ]; then
+if [ -z ${guests_disabled:-} ]; then
    guests_disabled="true"
    ynh_app_setting_set --app=$app --key=guests_disabled --value=$guests_disabled
 fi
 
 # Define $registration_shared_secret if not already defined
-if [ -z $registration_shared_secret ]; then
+if [ -z ${registration_shared_secret:-} ]; then
    registration_shared_secret=""
    ynh_app_setting_set --app=$app --key=registration_shared_secret --value=$registration_shared_secret
 fi
 
 # Load up registration variables
 registration=$(ynh_app_setting_get --app=$app --key=registration)
-if [ -z $registration ]; then
+if [ -z ${registration:-} ]; then
 	if [ "$registration_disabled" -eq "1" ]
 	then
 		really_enable_open_registration="--really-enable-open-registration"