Why does my MVC dropdownlist have a value selected despite the property value being NULL?
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I have a dropdownlist where a user can select a care provider. In the database the value for this is `null` however in the application, a value is selected. What do I need to do to have the `null` value appear as a blank? I thought this was the default behavior. I changed to using strongly typed lists in my view model instead of the viewbag and this may have broken at that time.

Here is the view markup:

    <div class="editor-label">
        @Html.LabelFor(model => model.PsychologistId, "Psychologist")
    </div>
    <div class="editor-field">
        @Html.DropDownListFor(x => x.PsychologistId, Model.ListPsychologists)
        @Html.ValidationMessageFor(model => model.PsychologistId)
    </div>