void Main()
{
    SqlMapper.AddTypeHandler (new MoneyTypeHandler ());
    using (var db = new SqlConnection("Server=(local); Integrated Security=SSPI")) {
        db.Open ();

        // Normal CLR types work fine when nullable
        db.Query<int?> ("SELECT 1").Single ().Dump ();
        db.Query<int?> ("SELECT NULL").Single ().Dump ();

        // Nullable custom type works correctly when used as a property
        db.Query<Foo> ("SELECT 1 AS Cost").Single ().Dump ();
        db.Query<Foo> ("SELECT NULL AS Cost").Single ().Dump ();

        // Fails when used as a single column scalar result
        db.Query<Money?> ("SELECT 1").Single ().Dump ();
        db.Query<Money?> ("SELECT NULL").Single ().Dump ();
    }  
}

public class MoneyTypeHandler : SqlMapper.TypeHandler<Money>
{
    public override void SetValue (IDbDataParameter p, Money value)
    {
        p.DbType = DbType.Int32;
        p.Value = value.Value;
    }

    public override Money Parse (object obj)
    {
        if (obj is DBNull) {
            throw new InvalidCastException ("Expected type int, got type DBNull");
        }
        return new Money ((int) obj);
    }
}

public struct Money
{
    private readonly int value;

    public Money (int value)
    {
        this.value = value;
    }

    public int Value { get { return value; } }
}

public class Foo
{
    public Money? Cost { get; set; }
}