xUnit: Can't assert exceptions

        var exception = await Assert.ThrowsAsync<ArgumentException>(() => _endpoint.GetReport("1.1.1.1", null));
        Assert.Equal("Api key shouldn't be empty.", exception.Message);

System.ArgumentException: Api key shouldn't be empty.

System.ArgumentException
Api key shouldn't be empty.

protected string ApiKey
    {
        get => _apiKey;
        init
        {
            if (string.IsNullOrWhiteSpace(value))
            {
                throw new ArgumentException("Api key shouldn't be empty.");
            }

            _apiKey = value;
        }
    }