Data Type Mismatch when passing DateTimePicker.Value

Question

I was struggling so much with something that I eventually found out how to fix, but I don't know the underlying cause. If I add a DateTimePicker (in this case, dtRecordBatchDate) to a form and the user doesn't change it's default date and time and I pass its DateTimePicker.Value to a SQL command using Parameters property, I get this error:

using (OleDbCommand command = new OleDbCommand("INSERT INTO Herbs_Stock (herb_id,batch_number,batch_date,quantity,cost,shipcost) VALUES (?,?,?,?,?,?)", Program.myconnection))
{
     command.Parameters.AddWithValue("@herb_id", herb_id);
     command.Parameters.AddWithValue("@batchnumber", batch_number);

     command.Parameters.AddWithValue("@batch_date", dtRecordBatchDate.Value); // HERE'S THE IMPORTANT PART

     command.Parameters.AddWithValue("@quantity", quantity);
     command.Parameters.AddWithValue("@cost", cost);
     command.Parameters.AddWithValue("@shipcost", shipcost);
     command.ExecuteNonQuery();
}

Data Type Mismatch in Criteria Expression

But if I add only one line before passing to the command it will be fixed: dtRecordBatchDate.Value=DateTime.Today.

Why is it like so? This is weird because type of DateTimePicker.Value is DateTime not DateTime? and it's got already defaulted to DateTime.Now (or a few seconds before :)). I'm very curious about this.