I have a mode which has a int? value

and I have a dropdown list which looks something like

@Html.DropDownListFor(model => model.ChildItems[i].RestrictionValue,
                                                               from Project.Core.Enums.Items.Restriction state in Enum.GetValues(typeof(Project.Core.Enums.Items.Restriction))
                                                               select new SelectListItem() { Text = state.ToString(), Value = ((int)state).ToString() }
                                                         ,"Select", Model.ChildItems[i].RestrictionValue
                                        )

but when there is no value select I get the error

Nullable object must have a value.

is there a way that the selected field can have a 0 value? or is there another work arround