diff --git a/backend/src/utils/validation.js b/backend/src/utils/validation.js
index 33ef16c..3bd4235 100644
--- a/backend/src/utils/validation.js
+++ b/backend/src/utils/validation.js
@@ -94,7 +94,7 @@ const userProductSchema = Joi.object({
       id: Joi.number().optional(), // For existing settings
       equipmentId: Joi.number().integer().positive().optional(), // Link to user_equipment
       // Legacy fields for backward compatibility
-      spreaderBrand: Joi.string().max(100).allow(null, undefined).optional(),
+      spreaderBrand: Joi.string().max(100).allow(null, '').optional(),
       spreaderModel: Joi.alternatives().try(
         Joi.string().max(100).allow(''),
         Joi.allow(null)