How to mapping identity fields?

Question

How to setup FieldValueMap for identityRef fields, AssignedTo for example? I migrate from DevOps Services to DevOps Server 2020 Update 1.1

// no work
{
      "$type": "FieldValueMapConfig",
      "WorkItemTypeName": "*",
      "sourceField": "System.AssignedTo.id",
      "targetField": "System.AssignedTo.id",
      "valueMapping": {
      }
}
// migration.exe crash
{
      "$type": "FieldValueMapConfig",
      "WorkItemTypeName": "*",
      "sourceField": "System.AssignedTo",
      "targetField": "System.AssignedTo",
      "valueMapping": {
    {
            "displayName": "displayName",
            "url": "url",
            "_links": {
                "avatar": {
                    "href": "href"
                }
            },
            "id": "id",
            "uniqueName": "uniqueName",
            "imageUrl": "imageUrl",
            "descriptor": "descriptor"
        }: {
        "displayName": "mapped_displayName",
            "url": "mapped_url",
            "_links": {
                "avatar": {
                    "href": "mapped_href"
                }
            },
            "id": "mapped_id",
            "uniqueName": "mapped_uniqueName",
            "imageUrl": "mapped_imageUrl",
            "descriptor": "mapped_descriptor"
        }
          }
    }

Can anybody help?

Same issue on github.