CallerMemberName doesn't return propertyName

Question

I'm working with some code that's suddenly stopped functioning. I tracked the problem down to CallerMemberName feature in .NET 4

It seems that it just stopped working, [CallerMemberName] String propertyName = "" returns "", even though it's being called from the property "speed". It was working fine previously, and I checked that the project is targeting ".NET Framework 4". Any idea what could cause this?

Property code:

    public double speed
    {
        get { return _speed; }
        set
        {
            _speed = value;
            NotifyPropertyChanged();
        }
    }

and it's processed here:

    //events
    public event PropertyChangedEventHandler PropertyChanged;

    public void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    void RealTimeDashboard_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        switch (e.PropertyName)
        {
            case "speed":
                NotifyPropertyChanged("speedDescription");
                CalculateEfficency();
                CheckEfficentConfigurations();
                break;
        }
    }

Edit: Installed The Microsoft BCL Portability Pack and it behaves the same but with the following warning:

Warning 10 The type 'System.Runtime.CompilerServices.CallerMemberNameAttribute' in 'D:\Projects...MainWindow.xaml.cs' conflicts with the imported type 'System.Runtime.CompilerServices.CallerMemberNameAttribute' in 'd:\Projects...\packages\Microsoft.Bcl.1.1.10\lib\net40\System.Runtime.dll'. Using the type defined in 'D:\Projects....\MainWindow.xaml.cs'. D:\Projects...AnOtherClass.xaml.cs