Can't save this dataflow due to dynamic data source

Does anybody know why I cannot save this dataflow?

What is a dynamic data source and where is it here?

let

Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WKi8v18tMytVLSVWK1YFwkzJzUsD8WAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [URL = _t]),

Custom = Table.AddColumn(Source, "TextFromBinaries", each Text.FromBinary(Web.Contents([URL])), type text)

in

Custom

 

Error message