MVC Dropdown list null on view on submit - asp.net-mvc

Question

I'm having issues with drop down lists on MVC. I've searched and searched but to no avail.
My ViewModel
public class IncidentFormViewModel
{
public Guid Guid { get; set; }
public Incident Incident { get; set; }
public Guid PersonInvolvedId { get; set; }
public IEnumerable<Person> People { get; set; }
}
My Controller
public ActionResult New()
{
var incidentFormVM = new IncidentFormViewModel
{
Incident = new Incident(),
People = unitofwork.Person.GetAll()
};
return View("IncidentForm", incidentFormVM);
}
and View(I've taken out useless information)
#using (Html.BeginForm("Save", "Incident"))
<div class="container">
<div class="row">
<div class="col-sm-6">
<div class="form-group">
#Html.LabelFor(m => m.Incident.PersonInvolved)
#Html.DropDownListFor(m => m.PersonInvolvedId, new SelectList(Model.People, "Id", "FirstName"), new { #class = "form-control" })
#Html.ValidationMessageFor(m => m.Incident.PersonInvolved)
</div>
</div>
<br />
<button type="submit" class="btn btn-primary">Submit</button>
</div>
</div>
The thing is if i put a stop on the line below and put a watch on Model.People i can see the list of people against it.
#Html.DropDownListFor(m => m.PersonInvolvedId, new SelectList(Model.People, "Id", "FirstName"), new { #class = "form-control" })
What am i doing wrong?