diff --git a/api/src/chat/validation-rules/is-message.ts b/api/src/chat/validation-rules/is-message.ts
index 0b11cc8f..fbd1589b 100644
--- a/api/src/chat/validation-rules/is-message.ts
+++ b/api/src/chat/validation-rules/is-message.ts
@@ -71,7 +71,7 @@ export function isValidMessage(msg: any) {
             .required(),
           payload: Joi.object().keys({
             url: Joi.string().uri(),
-            attachment_id: Joi.string(),
+            attachment_id: Joi.string().allow(null),
           }),
         }),
         elements: Joi.boolean(),