diff --git a/scripts/upgrade b/scripts/upgrade
index 655f49e..ef64b1e 100644
--- a/scripts/upgrade
+++ b/scripts/upgrade
@@ -21,7 +21,7 @@ upgrade_type=$(ynh_check_app_version_changed)
 ynh_script_progression --message="Ensuring downward compatibility..."
 
 # If database doesn't exist, create it and remove with_mysql setting
-if [ -z "$database" ]; then
+if [ -z "${database:-}" ]; then
     if [ $with_mysql -eq 1 ]; then
         database="mysql"
     else