Messenger bot with dialogflow does not respond to user

I have a messenger bot built with google's dialogflow engine, to respond the user I have a webhook. when I send a message with a "hello", the default welcome intent its triggered and the webhook it its called and it responds to me.

After that I need the products of my business so I write "Comprar" (Buy), the Products intent it is triggered and the webhook responds with a fulfillment message with a list of available products. In the history section in google dialogflow, I can see the json respond of the wekhook with the message that the user will receive.

But facebook does not show the message. I use the same webhook for many intents, and it is not working in this case.

this is the respond I got in the History section for this case:

{
  "id": "f75002cd-2f8a-422a-960d-dd2c9e00b490-74fe87bc",
  "fulfillmentText": "",
  "language_code": "es",
  "queryText": "Comprar",
  "webhookPayload": {},
  "intentDetectionConfidence": 1,
  "action": "GET_CATEGORIES",
  "webhookSource": "",
  "parameters": {},
  "fulfillmentMessages": [
    {
      "quickReplies": {
        "title": "Escoge alguna de estas Categorías:",
        "quickReplies": [
          "Volver",
          "",
          "FIRMA ELECTRONICA"
        ]
      }
    }
  ],
  "diagnosticInfo": {
    "webhook_latency_ms": "732.0"
  },
  "webhookStatus": {
    "webhookStatus": {
      "message": "Webhook execution successful"
    },
    "webhookUsed": true
  },
  "outputContexts": [
    {
      "lifespanCount": 1,
      "name": "vercatalogo-followup",
      "parameters": {}
    }
  ],
  "intent": {
    "isFallback": false,
    "displayName": "ver.catalogo",
    "id": "41dcecf2-11b6-4e50-8294-b86d849093e1"
  }
}

I solved lol

Apparently quickReplies must not contain a empty string ("") as a quick reply, that option is not valid. So I removed it and it worked.