当文本框的绑定设置为OneWayToSource时，WPF TargetNullValue返回值

I have this xaml textbox

我有这个xaml文本框

<TextBox Text="{Binding ProdFilter.Min, Mode=OneWayToSource,
   UpdateSourceTrigger=PropertyChanged, TargetNullValue=''}"
   Width="50" DockPanel.Dock="Right" TabIndex="3" />
<TextB