IConvertible for class-table in NET with MySQL db

Question

I use .NET with MySql , NPoco So two constructor : empty and copy. IConvertible is implemented. I successfully got data from another table with .Query But when I try get data from table Lesson in the line " // ERROR" - I got:

Object must implement IConvertible. - Run Time Error

(so I tried write full SQL query: select* from lesson where ... - same error) In MySQL FirstDate, LastDate have Date type. At that, field FromDate don't gives an error message. I will be grateful for help
Dependencies:

Connection to db:

_connectionString = "Server = 127.0.0.1; Database=test; user id=root; password=root;";
        _connection = new MySqlConnection(_connectionString);
        _connection.Open();



 [TableName("lessons")] 
[PrimaryKey("id")]
public class Lesson : IConvertible 
{
    [Column("id")]
    public int? LessonId 
{ get; 
set; }
    [Column("first_date")]
    public DateTime FirstDate 
{ get; 
set; }
    [Column("last_date")]
public DateTime LastDate
{ get; 
set; }  //  ERROR  here

public Lesson() 
    {
        Console.WriteLine("Constructor Lesson");
    } 

    public Lesson(Lesson torahLesson)
{
        LessonId = Lesson.LessonId;
        FirstDate = Lesson.FirstDate;
        LastDate = Lesson.LastDate;
}
 static T ThrowNotSupported<T>()
    {
        var ex = ThrowNotSupported(typeof(T));
        return (T)ex;
    }

    static object ThrowNotSupported(Type type)
    {
        throw new InvalidCastException($"Converting type \"{typeof(Lesson)}\" to type \"{type}\" is not supported.");
    }

    TypeCode IConvertible.GetTypeCode()
    {
        return TypeCode.Object;
    }
.....
    DateTime IConvertible.ToDateTime(IFormatProvider provider) => ThrowNotSupported<DateTime>();
 .....

}

Full Error Description :
at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider) at NPoco.MappingHelper.<>c__DisplayClass2_0.b__6(Object src) at NPoco.RowMappers.PropertyMapper.MapValue(GroupResult1 posName, Object[] values, Func2 converter, Object instance, PocoColumn pocoColumn, Object defaultValue) at NPoco.RowMappers.PropertyMapper.<>c__DisplayClass8_2.b__3(DbDataReader reader, Object[] values, Object instance) at NPoco.RowMappers.PropertyMapper.<>c__DisplayClass7_0.b__1(DbDataReader reader, Object[] values, Object instance) at NPoco.RowMappers.PropertyMapper.Map(DbDataReader dataReader, RowMapperContext context) at NPoco.MappingFactory.Map(DbDataReader dataReader, Object instance) at NPoco.Database.Read[T](Object instance, DbDataReader r, PocoData pd)+MoveNext() at NPoco.Database.QueryImp[T](T instance, Expression1 listExpression, Func2 idFunc, Sql Sql, PocoData pocoData)+MoveNext() at System.Collections.Generic.List1..ctor(IEnumerable1 collection) at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source) at NPoco.Linq.QueryProvider1.ToList() at Avidov.Library.Server.Database.DbHelper.Query[T](Expression`1 whereExpression) in E:\MyData\Projects\Comp\LibraryDotNet\Comp.Library\Comp.Library.Serve \Database\DbHelper.cs:line 169 at Comp.Library.Server.Database.Repositories.LessonRepository.GetNotDeletedLessons(Int32 organizationId, Int32 year) in E:\MyData\Projects\Comp\LibraryDotNet\Comp.Library\Comp.Library.Serve \Database\Repositories\LessonRepository.cs:line 76 at Comp.Library.Server.LibraryManager.LessonsNumberDaysChanged(Int32 organizationId, DateTime createdAfterDate, Int32 budgetYear) in E:\MyData\Projects\Comp\LibraryDotNet\Comp.Library\Comp.Library.Serve \LibraryManager.cs:line 81 at Comp.Library.Client.Program.Main(String[] args) in E:\MyData\Projects\Comp\LibraryDotNet\Comp.Library\Comp.Library.Client\Program.cs:line 18" string