Sum rows in power query

NikD Paste this in Advanced Editor in Power Query.

let
    Source = Table.FromRows (
        Json.Document (
            Binary.Decompress (
                Binary.FromText (
                    "xZA5CsMwEEWvIlQbebSgpZQTxRHWArLV2PExcv8IUiRgCO7SDf+/X7zZNgy8p9AzYAx3uMahuBAsCrmm0aHBlpZqTaTBe3cKNvAmxRd588mhMdcS3YKuLviWUQpEHNHgYy6PJwBTMypuXmwtNi2tYcoQbo6Lya72cs9TO7kWxLCfiJKE67MuVH3M/+EjJCW8/XN/AQ==",
                    BinaryEncoding.Base64
                ),
                Compression.Deflate
            )
        ),
        let
            _t = ( ( type nullable text ) meta [ Serialized.Text = true ] )
        in
            type table [ IssueDate = _t, CustomerName = _t, Sum = _t ]
    ),
    ChangedType = 
        Table.TransformColumnTypes (
            Source,
            { { "IssueDate", type date }, { "CustomerName", type text }, { "Sum", type number } }
        ),
    GroupedRows =
        Table.Group (
            ChangedType,
            { "IssueDate", "CustomerName" },
            { { "Sum", each List.Sum ( [Sum] ), type nullable number } }
        )
in
    GroupedRows