dropdownlist - selectedindex = -1 not setting control to no item selected

Not sure what I'm missing, my code is:
        ddlPropertyType.DataTextField = "Name";
        ddlPropertyType.DataValueField = "PropertyTypeId";
        ddlPropertyType.DataSource = DBHelper.GetPropertyTypes();
        ddlPropertyType.DataBind();
        ddlPropertyType.SelectedIndex = -1;
Yet when the page loads, the dropdownlist is defaulting to the first item in the control, even though no item has "Selected" in the rendered HTML.
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