Power Query CASE to SELECT from another Table

Hello - this will return the expected result.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Vcw5DsAgDETRu7imsCFkKbNvSk9kcf9rZKCI5MLF+xpZlYQcjTgvnvmm7JQ8OJWUAvNTUwDnsnyFOdXUgItNEVxtasGt/Ip/6sDdrnrwsGkAT5uE4QtHOX8=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t, ColumnX = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", Int64.Type}, {"Column2", type text}, {"ColumnX", type text}}),
    Result = Table.AddColumn ( 
        #"Changed Type", "CUSTOMERTYPE", each if [ColumnX] = "" then "DNE" else Table.SelectRows(
        Codes, 
        (x)=> 
            x[LookupValue]=Text.Start([ColumnX], 1)
        ){0}[Value], type text 
    )
in
    Result