I've tried this on ES/Kibana 5.1.2 and 5.2.1 and have had the same problem.
Using a fresh install, I go to Kibana's Dev Tools tab and enter:
PUT test
{
  "mappings": {
    "test": {
      "properties": {
        "field1": {
          "type": "text"
        },
        "created_at": {
          "type": "date"
        }
      }
    }
  }
}
I get the result:
{
  "acknowledged": true,
  "shards_acknowledged": true
}
I then add the new index to my Kibana indices:
Then I go to Kibana's Discover tab and see:
I've tried different mappings and different ES/Kibana version combinations. I'm at a loss of what could be wrong!
Please let me know if you need any additional information.