argumentnullexception with dropdownlistfor helper: value cannot be null

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MVC

C#5.0

Hi
i am binding data to dropdownlist with viewmodel dynamically.but while i submit data to server i am getting argumentnullexception.

data is binding to the dropdown but i am not to send the data to server

can you please solve my problem.

@Html.DropDownListFor(m => m.VerticalID, new SelectList(Model.naVertical, "VerticalID", "Name"), "Select Vertical")

public class NAVertical
  {
      public int VerticalID { get; set; }
      public string Name { get; set; }
  }

regards,
syed