Unable to show List selection - actions-on-google

Question

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!

Answer 1

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
}
}

Answer 2

One addition to the right answer by Chad: You aren't required to give an image url, but the Action must deliver the structure. So if you don't have an image with your list item, you can simply leave the url property blank:
{
"name": "ITEM_1",
"synonyms": [
"Item 1",
"Primer item"
],
"display": {
"title": "Artículo 1",
"description": "Descripción del artículo 1",
"image": {
"url": ""
}
}
}