RavenDb Index Output Issue

Failed to execute mapping function on contracts-8314. Exception: System.InvalidOperationException: The output of the mapping function does not contain all fields that the index is supposed to group by.
Output: { AccountId = accounts-994, ContractId = contracts-8314, DateCreated = 04/02/2019 15:03:47, DateLastModified = 02/24/2020 09:13:05 }
Group by fields: AccountId,ContractId
Compiled index def:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text.RegularExpressions;
using Lucene.Net.Documents;
using Raven.Client.Documents.Indexes;
using Raven.Server.Documents.Indexes.Static;
using Raven.Server.Documents.Indexes.Static.Linq;
using Raven.Server.Documents.Indexes.Static.Extensions;

namespace Raven.Server.Documents.Indexes.Static.Generated
{
    public class Index_Contracts_RefreshStats : StaticIndexBase
    {
        IEnumerable Map_0(IEnumerable<dynamic> docs)
        {
            foreach (var contract in docs)
            {
                yield return new
                {
                    AccountId = contract.AccountId,
                    ContractId = Id(contract),
                    DateCreated = contract.DateCreated,
                    DateLastModified = contract.DateLastModified
                }

                ;
            }
        }

        IEnumerable Map_1(IEnumerable<dynamic> docs)
        {
            foreach (var row in docs)
            {
                if (((bool)(row.ContractId != null) && (bool)(row.IsManagementRow == false)) == false)
                    continue;
                yield return new
                {
                    AccountId = row.AccountId,
                    ContractId = row.ContractId,
                    DateCreated = row.DateCreated,
                    DateLastModified = row.DateLastModified
                }

                ;
            }
        }

        public Index_Contracts_RefreshStats()
        {
            this.AddMap("Contracts", this.Map_0);
            this.AddMap("IncomeRows", this.Map_1);
            this.Reduce = results => results.GroupBy((Func<dynamic, dynamic>)(item => new
            {
                AccountId = item.AccountId,
                ContractId = item.ContractId
            }

            )).Select((Func<IGrouping<dynamic, dynamic>, dynamic>)(gr => new
            {
                AccountId = gr.Key.AccountId,
                ContractId = gr.Key.ContractId,
                DateCreated = DynamicEnumerable.Max(gr, (Func<dynamic, DateTime>)(x => ((DateTime)x.DateCreated))),
                DateLastModified = DynamicEnumerable.Max(gr, (Func<dynamic, DateTime>)(x0 => ((DateTime)x0.DateLastModified)))
            }

            ));
            this.GroupByFields = new Raven.Server.Documents.Indexes.CompiledIndexField[] { new Raven.Server.Documents.Indexes.SimpleField("AccountId"), new Raven.Server.Documents.Indexes.SimpleField("ContractId") };
            this.OutputFields = new System.String[] { "AccountId", "ContractId", "DateCreated", "DateLastModified" };
        }
    }
}
   at Raven.Server.Documents.Indexes.MapReduce.Static.MapReduceIndex.AnonymousObjectToBlittableMapResultsEnumerableWrapper.Enumerator.ThrowMissingGroupByFieldsInMapOutput(Object output, HashSet`1 groupByFields, StaticIndexBase compiledIndex) in C:\Builds\RavenDB-Stable-4.2\42009\src\Raven.Server\Documents\Indexes\MapReduce\Static\MapReduceIndex.cs:line 486
   at Raven.Server.Documents.Indexes.MapReduce.Static.MapReduceIndex.AnonymousObjectToBlittableMapResultsEnumerableWrapper.Enumerator.MoveNext() in C:\Builds\RavenDB-Stable-4.2\42009\src\Raven.Server\Documents\Indexes\MapReduce\Static\MapReduceIndex.cs:line 459
   at Raven.Server.Documents.Indexes.MapReduce.MapReduceIndexBase`2.PutMapResults(LazyStringValue lowerId, LazyStringValue id, IEnumerable`1 mappedResults, TransactionOperationContext indexContext, IndexingStatsScope stats) in C:\Builds\RavenDB-Stable-4.2\42009\src\Raven.Server\Documents\Indexes\MapReduce\MapReduceIndexBase.cs:line 121
   at Raven.Server.Documents.Indexes.MapReduce.Static.MapReduceIndex.HandleMap(LazyStringValue lowerId, LazyStringValue id, IEnumerable mapResults, IndexWriteOperation writer, TransactionOperationContext indexContext, IndexingStatsScope stats) in C:\Builds\RavenDB-Stable-4.2\42009\src\Raven.Server\Documents\Indexes\MapReduce\Static\MapReduceIndex.cs:line 287
   at Raven.Server.Documents.Indexes.Workers.MapDocuments.Execute(DocumentsOperationContext databaseContext, TransactionOperationContext indexContext, Lazy`1 writeOperation, IndexingStatsScope stats, CancellationToken token) in C:\Builds\RavenDB-Stable-4.2\42009\src\Raven.Server\Documents\Indexes\Workers\MapDocuments.cs:line 110