MvvmLight ToolKit .Net4.5版本 CanExecute不能刷新界面bug

　　一问题重现

　 1.在使用最新版本v5.1的MvvmLight中（其实这个问题很早就有了），发现CanExecute不能很好地工作了。一个简单的工程，只有MainWindow和MainWindowViewModel,代码如下

MainWindow.xaml

<Window x:Class="MvvmLightError.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Button Width="100" Height="30" Content="OK" Command="{Binding OkCommand}"></Button>

        <Button Width="100" Height="30" Margin="0,100,0,0" Content="ChangeEnable" Command="{Binding ChangeEnableCommand}"></Button>
    </Grid>
</Window>

MainWindow.xaml.cs

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            this.DataContext = new MainWindowViewModel();
        }
    }

MainWindowViewModel.cs

public class MainWindowViewModel : ViewModelBase
    {
        private bool _isEnabled;

        public RelayCommand OkCommand
        {
            get;
            set;
        }

        public RelayCommand ChangeEnableCommand
        {
            get;
            set;
        }

        public MainWindowViewModel()
        {
            OkCommand = new RelayCommand(OkCommandHanlder, CanOkCommandExecute);

            ChangeEnableCommand = new RelayCommand(ChangeEnableCommandHandler);
        }

        private void OkCommandHanlder()
        {

        }

        private bool CanOkCommandExecute()
        {
            return _isEnabled;
        }

        private void ChangeEnableCommandHandler()
        {
            _isEnabled = !_isEnabled;
        }
    }