DataRow.Field<T>(string Column) кидает ошибку преобразования

Есть вот такой метод:

 Dictionary<string, string> GetColumnsList(OleDbConnection conn, string tableName)
        {
            var columns = conn.GetOleDbSchemaTable(OleDbSchemaGuid.Columns, new[] { null, null, tableName, null });
            return columns.AsEnumerable().
                Select(x => new
                {
                    ColumnName = x.Field<string>("Column_Name"),
                    Description = $"Data_Type:{ x.Field<int>("Data_Type")};"+
                                  $"IsNullable:{ x.Field<bool>("IS_NULLABLE")};"+
                                  $"CharacterMaximumLength: { x.Field<int?>("Character_Maximum_Length")};"+
                                  $"CharacterOctetLength: { x.Field<int?>("Character_Octet_Length")}"
        }).ToDictionary(key => key.ColumnName, value => value.Description);
        }

Так вот, не могу понять, почему вылетает исключение при попытке использовать int?.

Сами по себе колонки в DataTable имеют тип int.

System.InvalidCastException: 'Specified cast is not valid.'

at System.Data.DataRowExtensions.UnboxT1.NullableField[TElem](Object value) at System.Data.DataRowExtensions.Field[T](DataRow row, String columnName) at AccessToMySql.Classes.AccessReader.<>c.<GetColumnsList>b__5_0(DataRow x) in D:\Projects\AccessToMySql\AccessToMySql\Classes\AccessReader.cs:line 52 at System.Linq.Enumerable.WhereSelectEnumerableIterator2.MoveNext()
at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable1 source, Func2 keySelector, Func2 elementSelector, IEqualityComparer1 comparer) at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable1 source, Func2 keySelector, Func`2 elementSelector) at AccessToMySql.Classes.AccessReader.GetColumnsList(OleDbConnection conn, String tableName) in D:\Projects\AccessToMySql\AccessToMySql\Classes\AccessReader.cs:line 51 at AccessToMySql.Classes.AccessReader.ProcessingMdb(String pathToMdb) in D:\Projects\AccessToMySql\AccessToMySql\Classes\AccessReader.cs:line 37 at AccessToMySql.Classes.AccessReader.Read() in D:\Projects\AccessToMySql\AccessToMySql\Classes\AccessReader.cs:line 26 at AccessToMySql.Program.Main(String[] args) in D:\Projects\AccessToMySql\AccessToMySql\Program.cs:line 17