creating subscription with Microsoft Graph API error

I'm trying to create a subscription to receive notification about changes to Office365 Calendar. I have already worked with Outlook Notitication API but having a problem with Microsoft Grap API

I try to create a subscription using Http Post to https://graph.microsoft.com/beta/subscriptions with header and body:

• Header: Content-Type: application/json; Authorization : Bearer {accessToke}

• Body:

  {
  "resource": "me/calendars",
  "notificationUrl": "sample notification url",
  "changeType": "Created", }

In my notification url, i've setup to send a response with the value of validation token when receveving a validation request

Finally, the result i received:

{
"error": {
"code": "ExtensionError",
"message": "There was an error processing a storage extension.",
"innerError": {
"request-id": "6c563931-511d-415d-9c04-c07f25d45b1f",
"date": "2016-03-20T08:10:32"
}
}
}

I wonder what I'm doing wrong or that's a internal error of MS Grap API. Can anyone help me? Thank in advance