Use Dapper to insert null from"DateTime?" into PostgreSql timestamp

Question

I'am using .net core. I have following entity, where EndDate could be null:

public class MyEntity
{
    public Guid Id { get; set; }
    public DateTime StartDate { get; set; }
    public DateTime? EndDate { get; set; }
}

My table:

create table MyEntity(
    id uuid not null,
    start_date timestamp not null,
    end_date timestamp
    constraint MyEntity_pkc primary key (id)
);

I have following merge statement:

merge into MyEntity c 
using (values (@Id, @StartDate, @EndDate)) v
on c.id = v.column1
when not matched then
insert (id, start_date, end_date)
values (v.column1, v.column2, v.column3)
when matched then
update set start_date = v.column2, end_date = v.column3

I execute merge, where entity.EndDate == null:

await connection.ExecuteAsync(sqlTemplate.Get("MergeMyEntity"), entity);

I've got error:

Npgsql.PostgresException (0x80004005): 42804: column "end_date" is of type timestamp without time zone but expression is of type text

How should I insert null for EndDate?