绑定WPF int和int？

Question

I have template column like this. When I set property in other column I regenerate columns and invoke UpdateLayout for a grid and after that my combobox doesn't bind property of int but when this property is int? all working ok. Please help me to explain this behaviour.

<TemplateColumnMetadata Name="User" Header="User"  MinWidth="150">
    <TemplateColumnMetadata.ColumnTemplate>
        <DataTemplate>
            <ComboBox    
                ItemsSource="{Binding Model.Users}"
                DisplayMemberPath="Name"
                SelectedValuePath="Id"
                SelectedValue="{Binding UserId, UpdateSourceTrigger=PropertyChanged}"       
                IsEnabled="{Binding IsCellEditable}">
            <ComboBox>
        </DataTemplate>
    <TemplateColumnMetadata.ColumnTemplate>           
<TemplateColumnMetadata>

1. This code works

public int? UserId
{
    get { return Model.UserId; }
    set
    {
        Model.UserId= value.Value;
    }
}

2. This code doesn't work

public int UserId
{
    get { return Model.UserId; }
    set
    {
        Model.UserId= value;
    }
}

Answer 1

The problem occured because null value went from xaml and binding got broken. So it worked for int? but it didn't work for int.