Command binding - CanExecuteChanged seemingly not firing (using CommandReference)

<Windowx:Class="CommandBinding.MainWindow"xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"xmlns:local="clr-namespace:CommandBinding"Title="MainWindow"Width="200"><Window.Resources><local:CommandReferencex:Key="SubmitCommand"Command="{Binding Path=SubmitCommand, UpdateSourceTrigger=PropertyChanged}"/><local:CommandReferencex:Key="RequestCommand"Command="{Binding Path=RequestCommand, UpdateSourceTrigger=PropertyChanged}"/><local:CommandCollectionx:Key="SubmitCommandCollection"><local:CommandReferenceCommand="{StaticResource ResourceKey=SubmitCommand}"/><local:CommandReferenceCommand="{StaticResource ResourceKey=RequestCommand}"/></local:CommandCollection></Window.Resources><StackPanel><ButtonCommand="{Binding Path=RequestCommand}"Margin="10, 10">Request</Button><ButtonCommand="{StaticResource ResourceKey=SubmitCommandCollection}"Margin="10, 0">Submit</Button><TextBoxText="{Binding Path=Number, Mode=OneWay}"IsReadOnly="True"Margin="10"TextAlignment="Center"/></StackPanel></Window>

using System;using System.Windows.Input;namespace CommandBinding.ViewModel
{publicclass ExampleViewModel : ViewModelObject
  {#region Fieldsprivateint _Number;private ICommand _SubmitCommand;private ICommand _RequestCommand;#endregion#region Propertiespublicint Number
    {get { return _Number; }set
      {if (_Number != value)
        {
          _Number = value;
          OnPropertyChanged("Number");
        }
      }
    }public ICommand SubmitCommand
    {get
      {if (_SubmitCommand == null)
        {
          _SubmitCommand = new RelayCommand<object>(RunSubmitCommand, CanRunSubmitCommand);
        }return _SubmitCommand;
      }
    }public ICommand RequestCommand
    {get
      {if (_RequestCommand == null)
        {
          _RequestCommand = new RelayCommand<object>(RunRequestCommand);
        }return _RequestCommand;
      }
    }#endregion#region Methdosprivatevoid RunSubmitCommand(object parameter)
    {
      Number = 0; //Simulate submitting the number and clearing
    }privatebool CanRunSubmitCommand(object parameter)
    {//Number = 100; //NOTE: If you uncomment this line, you'll see that the submit button will always be enabled, otherwise it will never be enabledreturn Number > 50;
    }privatevoid RunRequestCommand(object parameter)
    {
      Number = new Random().Next(1, 100);//CommandManager.InvalidateRequerySuggested(); //This seemingly does nothing//OnPropertyChanged("SubmitCommand"); //This also doesn't seem to help
    }#endregion
  }
}