PropertyChanged not raised when TextBox bound to a null property becomes null

I have this field and property

private long? _defautID;
public long? DefautID
{
    get => _defautID;
    set
    {
        if (_defautID != value)
        {
            _defautID = value;
            OnPropertyChanged();
        }
    }
}

linked to a textbox

<TextBox Content="{Binding 
                   Path=Expertise.DefautID, UpdateSourceTrigger=PropertyChanged}"/>

When I add or remove a digit from the textbox, my property is updated and PropertyChanged is triggered without any problem.

But when I remove the last digit and my textbox becomes empty, my property is not updated (it keeps the last known value) and thus PropertyChanged is not triggered.

Is there any reason for this behavior ? And is there a workaround ?

PropertyChanged implementation :

public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}

Set the TargetNullValue property of the binding:

<TextBox Text="{Binding 
    Path=Expertise.DefautID, UpdateSourceTrigger=PropertyChanged, TargetNullValue=''}"/>

This is required for the emtpy string in the TextBox to be successfully converted to default(long?) and for your property to be set.