Grid resources not working. What to do?

Question

WPF

I created several classes with my own methods, but I can't connect them. Here is my code:

using System;
using System.Globalization;
using System.Windows.Data;

namespace WeirdShit
{
    public class Bool2String : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            bool what = (bool)value;
            if (what)
                return Properties.Settings.Default.FilledHeart;
            else
                return Properties.Settings.Default.EmptyHeart;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value.ToString().Contains(Properties.Settings.Default.EmptyHeart))
                return value;
            else
                return true;
        }
    }

I'm trying to connect:

<Grid.Resources>
            <local:Bool2String x:Key="star"></local:Bool2String>
        </Grid.Resources>

Gives the error "the name doesn't exist in namespace". I've checked everything and can't find the error. Help me please