I don't know why I'm getting this error on Task 1 of 4 of TDD section of C# Unit Testing course

using System;

public class Calculator
{

    public double Result;

    public Calculator(double value)
    {
        Result = value;
    }

    public void Initialization()
    {

    }

    public double BasicAdd(double value)
    {
       return Result += value;
    }
}

using Xunit;


public class CalculatorTests
{
    [Fact]
    public void Initialization()
    {
        var expected = 1.1;
        var target = new Calculator(1.1);
        Assert.Equal(expected, target.Result, 1);
    }

    [Fact]
    public void BasicAdd()
    {
        var target = new Calculator(1.1);
        target.Add(2.2);
        var expected = 3.3;
        Assert.Equal(expected, target.Result, 1);
    }
}