One to One mapping and join two condition, how to set splitOn

my sql is like this
string sql = SELECT * FROM B JOIN A ON B.ColumnD = A.ColumnD AND B.ColumnE = A.ColumnE

class TestA
{
    public string ColumnD { get; set; }
    public string ColumnE { get; set; }
    public string ColumnF { get; set; }
    public string ColumnG { get; set; }
}

class TestB
{
    public string ColumnA { get; set; }
    public string ColumnB { get; set; }
    public string ColumnC { get; set; }
    public string ColumnD { get; set; }
    public string ColumnE { get; set; }
    public TestA Item { get; set; }
}

this is one to one mapping result

#IEnumerable<TestB> GetTestB()
{
    return Connection.Query<TestB, TestA, TestB>(
       sql,
        (b, a) =>
        {
            b.Item = a;
            return b;
        },
        splitOn: "ColumnD,ColumnE");
}

but the result TestA always NULL
splitOn setting wrong?

Start with the Connection.Query<TestB, TestA, TestB> call, the SQL JOIN, and the TestA/TestB class definitions shown in the issue. Reproduce the one-to-one mapping using the current splitOn value and inspect which returned columns are used to construct TestA. Done means determining the correct mapping behavior and documenting or testing the expected non-null TestA result.