Expression.Error: We cannot convert a value of type Table to type List. Details: Value=[Table]

Hello, I cannot figure out where my error lies. Admittedly, I am pretty new to PowerBI... Many thanks in advance! 

let
source1 = Master1,
source2 = AAA_Grants,
mappingTable = #"AAA_to_CAF_col_map",

// Step 1: Rename specific columns in Table "AAA_Grants" using the mapping
renamedAAA_Grants = Table.RenameColumns(source2, Table.Transform(Table.ColumnNames(source2), each {_ , Table.First(Table.SelectRows(mappingTable, each [OldColumnName] = _))[NewColumnName]})),

// Step 2: Merge Table "AAA_Grants" with Table "Master1" and keep all columns from both tables
mergedTable = Table.NestedJoin(renamedAAA_Grants, {"CommonColumn"}, source1, {"CommonColumn"}, "NewColumn"),
expandedTable = Table.ExpandTableColumn(mergedTable, "NewColumn", Table.Union(Table.ColumnNames(renamedAAA_Grants), Table.ColumnNames(source1)))

in
expandedTable