Unable to show List selection

I'm developing an Action where I want to display a list of items to select using a List, but Assistant raises the following error:

Unexpected internal error id=83ef0935-3d8f-473d-9e66-f96c886cd4cd.

This is my Scene:

I have created a empty Type called prompt_option. The webhookResponse is the following:

{
  "responseJson": {
    "prompt": {
      "firstSimple": {
        "speech": "Vale, te muestro los últimos artículos"
      },
      "content": {
        "list": {
          "title": "Últimos artículos",
          "items": [
            {
              "key": "ITEM_1"
            },
            {
              "key": "ITEM_2"
            },
            {
              "key": "ITEM_3"
            }
          ]
        }
      }
    },
    "scene": {
      "name": "Articles",
      "slotFillingStatus": "COLLECTING",
      "slots": {
        "prompt_option": {
          "mode": "REQUIRED",
          "status": "SLOT_UNSPECIFIED",
          "updated": false
        }
      }
    },
    "session": {
      "id": "[...]" ,
      "params": {
        "choosen_option": "Portada"
      },
      "typeOverrides": [
        {
          "name": "prompt_option",
          "mode": "TYPE_REPLACE",
          "synonym": {
            "entries": [
              {
                "name": "ITEM_1",
                "synonyms": [
                  "Item 1",
                  "Primer item"
                ],
                "display": {
                  "title": "Artículo 1",
                  "description": "Descripción del artículo 1"
                }
              },
              {
                "name": "ITEM_2",
                "synonyms": [
                  "Item 2",
                  "Segundo item"
                ],
                "display": {
                  "title": "Título del artículo 2",
                  "description": "Resumen del artículo 2"
                }
              },
              {
                "name": "ITEM_3",
                "synonyms": [
                  "Item 3",
                  "Tercer item"
                ],
                "display": {
                  "title": "Título del artículo 3",
                  "description": "Resumen del artículo 3"
                }
              }
            ]
          }
        }
      ],
      "languageCode": ""
    },
    "user": {
      "locale": "es-ES",
      "params": {},
      "accountLinkingStatus": "ACCOUNT_LINKING_STATUS_UNSPECIFIED",
      "verificationStatus": "VERIFIED",
      "packageEntitlements": [],
      "gaiamint": "",
      "lastSeenTime": "2021-01-05T15:14:30Z"
    },
    "home": {
      "params": {}
    },
    "device": {
      "capabilities": [
        "SPEECH",
        "RICH_RESPONSE",
        "LONG_FORM_AUDIO"
      ]
    }
  }
}

I can't figure what I'm doing wrong. Any advice? Thanks!

While the property is clearly defined as optional, the display property of the type override requires an image.

{
  "name": "ITEM_1",
  "synonyms": [
    "Item 1",
    "Primer item"
  ],
  "display": {
    "title": "Artículo 1",
    "description": "Descripción del artículo 1",
    "image": IMAGE_REQUIRED
  }
}