diff --git a/src/CL/Slack/Resources/config/serializer/CL.Slack.Model.Attachment.yml b/src/CL/Slack/Resources/config/serializer/CL.Slack.Model.Attachment.yml
index 89ffe5a3..b72b77fa 100644
--- a/src/CL/Slack/Resources/config/serializer/CL.Slack.Model.Attachment.yml
+++ b/src/CL/Slack/Resources/config/serializer/CL.Slack.Model.Attachment.yml
@@ -14,3 +14,5 @@ CL\Slack\Model\Attachment:
             type: string
         fields:
             type: ArrayCollection<CL\Slack\Model\AttachmentField>
+        mrkdwnIn:
+            type: array