Azure Search Custom Skill API Invalid Response

I'm implementing an Azure Cognitive Search Skillset and I'm at my wits end with this issue:

The only skill I currently have in the skillset is a custom skill that calls an app function.

The app function is defined as follows:

        [FunctionName("TestFunction")]
        public static async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,
            ILogger log)
        {
            object result = new
            {
                Values = new List<object>()
                {
                    new
                    {
                        RecordId = "b5",
                        Data = new
                        {
                            ContractDate = new 
                            {
                                Day = 5,
                                Month = 2,
                                Year = 2018
                            }
                        }
                    },
                    new
                    {
                        RecordId = "ai",
                        Data = new
                        {
                            ContractDate = new
                            {
                                Day = 3,
                                Month = 11,
                                Year = 2017
                            }
                        }
                    },
                    new
                    {
                        RecordId = "c3",
                        Data = new { },
                        Errors = new List<object>()
                        {
                            new 
                            {
                                Message = "contractText field required "
                            }
                        },
                        Warnings = new List<object>()
                        {
                            new
                            {
                                Message = "Date not found"
                            }
                        }
                    }
                }
            };

            return new OkObjectResult(result);
        }
    }

The skillset is defined as:

{
  "@odata.context": "https://<ommitted>.search.windows.net/$metadata#skillsets/$entity",
  "@odata.etag": "\"0x8DBCF46B8B13B2B\"",
  "name": "testskillset",
  "description": "Skillset for testing purposes",
  "skills": [
    {
      "@odata.type": "#Microsoft.Skills.Custom.WebApiSkill",
      "name": "custom_skill_1",
      "description": "",
      "context": "/document",
      "uri": "https://<ommitted>.azurewebsites.net/api/TestFunction?code=<ommitted>",
      "httpMethod": "POST",
      "timeout": "PT30S",
      "batchSize": 1000,
      "degreeOfParallelism": null,
      "inputs": [
        {
          "name": "file_url",
          "source": "/document/file_data/url"
        }
      ],
      "outputs": [
        {
          "name": "customapiresult",
          "targetName": "custom_api_result"
        }
      ],
      "httpHeaders": {}
    }
  ],
  "cognitiveServices": {
    "@odata.type": "#Microsoft.Azure.Search.DefaultCognitiveServices",
    "description": null
  },
  "knowledgeStore": null,
  "encryptionKey": null
}

When I publish the function to Azure and call it from Postman, it works as expected and returns the following result:



That result fits the format requirement for a custom skill exactly according to this article.

Unfortunately, when I run the skillset in a cognitive search debug session, I get the following error:

All three records that are returned, have id's so I have no clue what the problem is.

Thanks in advance.