This M (Power Query) function gives this error “[Expression.Error] We cannot convert a value of type Function to type List.” The objective is to output a table column being the difference between two source columns.

For Example:
• 1, 667

let
  Source = Table.FromRecords({
        [row = 1, first = "1234", second="567"],
        [row = 2, first = "9876", second="543"],
        [row = 3, first = "5437", second="890"],
        [row = 4, first = "2321", second="550"]
    }),
  Source2 = Table.FromColumns(
    {Source[#"row"], each Source[#"first"] - Source[#"second"]},{"row","diff"})
in
  Source2

Could anyone please point out the error? I am trying to subtract one existing column from another and put the result in a new column