How to get list of files in root folder or in any folder with API v2

Using the example below:

    client.files.listFolder(path: "").response { response, error in

                if let result = response {

                    print("Folder contents:")

                    for entry in result.entries {

                        print(entry.name)

 

returns nothing (or rather nil for the response var), and understanding this represents ROOT folder, and I get this error:

Bad Input: Error in call to API function "files/list_folder": request body: could not decode input as JSON

So ... I was able to upload a file, so the token is good, but any list, either files or shared, returns nothing.  Guessing something not set correct either in the account or this api ... thanks for your help !