diff --git a/src/routes/validate/sample.ts b/src/routes/validate/sample.ts
index 9c5ad0d..54f7bb4 100644
--- a/src/routes/validate/sample.ts
+++ b/src/routes/validate/sample.ts
@@ -209,7 +209,7 @@ export default class SampleValidate {
             if (/material\./.test(field)) {  // select right validation model
               validator = MaterialValidate.outputV().append({
                 number: Joi.string().max(128).allow(''),
-                properties: Joi.alternatives().try(Joi.number(), Joi.string().max(128))
+                properties: Joi.alternatives().try(Joi.number(), Joi.string().max(128).allow(''))
               });
               field = field.replace('material.', '').split('.')[0];
             }
@@ -218,7 +218,7 @@ export default class SampleValidate {
                 value: Joi.alternatives()
                   .try(
                     Joi.number(),
-                    Joi.string().max(128),
+                    Joi.string().max(128).allow(''),
                     Joi.boolean(),
                     Joi.array()
                   )