diff --git a/scripts/upgrade b/scripts/upgrade
index 24876ee..a67c125 100755
--- a/scripts/upgrade
+++ b/scripts/upgrade
@@ -27,19 +27,19 @@ upgrade_type=$(ynh_check_app_version_changed)
 ynh_script_progression --message="Ensuring downward compatibility..." --weight=1
 
 # If fpm_footprint doesn't exist, create it
-if [ -z "$fpm_footprint" ]; then
+if [ -z "${fpm_footprint:-}" ]; then
     fpm_footprint=low
     ynh_app_setting_set --app=$app --key=fpm_footprint --value=$fpm_footprint
 fi
 
 # If fpm_free_footprint doesn't exist, create it
-if [ -z "$fpm_free_footprint" ]; then
+if [ -z "${fpm_free_footprint:-}" ]; then
     fpm_free_footprint=0
     ynh_app_setting_set --app=$app --key=fpm_free_footprint --value=$fpm_free_footprint
 fi
 
 # If fpm_usage doesn't exist, create it
-if [ -z "$fpm_usage" ]; then
+if [ -z "${fpm_usage:-}" ]; then
     fpm_usage=low
     ynh_app_setting_set --app=$app --key=fpm_usage --value=$fpm_usage
 fi