API Version 2: Failed to parse JSON response string with 'INVALID_ARGUMENT' error: \": Cannot find field.\"

Question

I am getting 'API Version 2: Failed to parse JSON response string with 'INVALID_ARGUMENT' error: \": Cannot find field.\".' error for simple speech webhook response.

------------ERROR--------------

"debugInfo": {
        "agentToAssistantDebug": {
            "agentToAssistantJson": {
                "message": "Unexpected apiai response format: Empty speech response",
                "apiResponse": {
                    "id": "31f9c31d-3861-4262-8518-bd1f1e895f86",
                    "timestamp": "2017-07-29T22:09:23.971Z",
                    "lang": "en",
                    "result": {},
                    "status": {
                        "code": 200,
                        "errorType": "success"
                    },
                    "sessionId": "1501366152335"
                }
            }
        },
        "sharedDebugInfo": [
            {
                "name": "ResponseValidation",
                "subDebugEntry": [
                    {
                        "name": "UnparseableJsonResponse",
                        "debugInfo": "API Version 2: Failed to parse JSON response string with 'INVALID_ARGUMENT' error: \": Cannot find field.\"."
                    }
                ]
            }
        ]
    },
    "visualResponse": {}
}

I tried sending following json response as per https://api.ai/docs/reference/agent/query#response document.

------------RESPONSE--------------

{
  "result": {
    "source": "agent",
    "resolvedQuery": "city",
    "action": "tell.facts",
    "actionIncomplete": false,
    "parameters": {
      "facts-category": "city"
    },
    "contexts": [],
    "metadata": {
      "intentId": "873b1895-cdfc-42a4-b61b-5a1703c72a4d",
      "webhookUsed": "true",
      "webhookForSlotFillingUsed": "false",
      "webhookResponseTime": 417,
      "intentName": "tell-facts"
    },
    "fulfillment": {
      "speech": "Amsterdam",
      "messages": [
        {
          "type": 0,
          "speech": "Amsterdam"
        }
      ]
    },
    "score": 1
  }
}

What is it that i'm missing??

Answer 1

In my case I forget to enable "Use webhook" in Fulfillment and enable "End conversation" in Google Assistant.