Необработанное исключение в BinarySearch

BindingList<D1> bl1 = new BindingList<D1>(); int Res = -2; Int16? FindVal = 0;
//поле D1.x0 имеет тип <Int16?>
if (bl1.Count > 0) Res = bl1.OrderBy(x => x.x0).ToList().BinarySearch(new D1 { x0 = FindVal });

if (!checkBox1.Checked) bl1 = new BindingList<D1>(bl1.OrderBy(x => x.x0).ToList());
else bl1 = new BindingList<D1>(bl1.OrderByDescending(x => x.x0).ToList());

dataGridView.DataSource = null;
dataGridView.DataSource = bl1;

    class D1 : IComparable
    {
        public Int16? x0 { get;set; }
 
        public int CompareTo(object obj)
        {
            var other = obj as D1;
            if (other == null) return 1;
            if (other.x0 == null && x0 == null) return 0;
            if (x0 == null) return -1;
            if (other.x0 == null) return 1;
            return x0.Value.CompareTo(other.x0.Value);
        }
    }

Res = bl1.IndexOf(new D1 { x0 = FindVal });

private Int16? X0;
 
public D1(Int16? x0) { this.X0 = x0; }
public D1() { this.X0 = null; }
public Int16? x0 { set { X0 = value; } get { return X0; } }
public int CompareTo(object obj) {...}

public bool Equals(D1 Other) {
    if (x0 != null && Other != null && Other.x0 != null) return Equals(Other);
    else return false;
}
public override bool Equals(Object obj) {
    var Other = obj as D1;
    if (x0 != null && Other != null && Other.x0 != null) return Equals(Other);
    else return false;
}
public override int GetHashCode() { return this.x0.GetHashCode(); }

public bool Equals(D1 Other) {
    if (x0 != null && Other != null && Other.x0 != null) return x0 == Other.x0;
    else return false;
}

public bool Equals(D1 Other) { if (x0 == Other.x0) return true; else return false; }

public bool Equals(D1 Other) { return x0 == Other.x0; }