html.dropdown list works for create but fails for edit

using dictionary<int, string> in viewmodel:
   public Dictionary<int, string> VehicleList { get; set; }

using dropdown list in view:  
   @Html.DropDownListFor(model => model.VehicleId, new SelectList(Model.VehicleList, "Key", "Value"), string.Empty, new { @class = "form-control" })

this works perfectly in the Create view, but fails in the the Edit view with this error:
System.ArgumentNullException occurred   HResult=0x80004003   Message=Value cannot be null.

I don't know how to correct this.  The definition of vehicleid in the the viewmodel is:  public int? VehicleId { get; set; }

any help appreciated!
dan hendersonAsked:
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Gautham JanardhanCommented:
Make sure VehicleList is not null in edit mode.

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dan hendersonAuthor Commented:
how exactly do I do that based on the code I provided?
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