diff --git a/scripts/upgrade b/scripts/upgrade
index 5ce62ed..69cea6a 100644
--- a/scripts/upgrade
+++ b/scripts/upgrade
@@ -28,7 +28,7 @@ upgrade_type=$(ynh_check_app_version_changed)
 ynh_script_progression --message="Ensuring downward compatibility..." --weight=1
 
 # If backend_path doesn't exist, create it
-if [ -z "$backend_path" ]; then
+if [ -z "${backend_path:-}" ]; then
 	backend_path=/opt/$app
 	ynh_app_setting_set --app=$app --key=backend_path --value=$backend_path
 fi