EventGrid Trigger and Publish Topic Action

I gave Flow and Azure Event Grid a spin this morning:

1. Created Event Grid

2. Was able to publish events via Flow to Event Grid Topic.

3. Was unable to trigger Flow from Event Grid - any attempt to set up the trigger results in a 

"id": "/subscriptions/44331b86-e755-4089-add0-2fd4c73c9725/resourceGroups/GraphEventGridResource/providers/Microsoft.EventGrid/topics/GraphEventGrid2/providers/Microsoft.EventGrid/eventSubscriptions/LogicApp0fb94caa-470f-4df4-8478-ee4f943a5fd0/events/839d55af-95d3-48c8-933c-295d904223c6/ticks/636424506192557074",
    "level": "Error",
    "operationId": "3108214e-52e7-4219-8b4e-66631be35859",
    "operationName": {
        "value": "Microsoft.EventGrid/eventSubscriptions/write",
        "localizedValue": "Microsoft.EventGrid/eventSubscriptions/write"
    },

    "properties": {
        "statusCode": "BadRequest",
        "serviceRequestId": null,
        "statusMessage": "\"Model state is invalid.\"" 
    },

Azure Event Log.