API raises exception when uploading big files in session

Hi, I have been trying for a few days to upload a big file into my Dropbox account using the python API,

i have tried using the following code:

f = open(file_path)
file_size = os.path.getsize(file_path)

CHUNK_SIZE = 4 * 1024 * 1024

if file_size <= CHUNK_SIZE:

    print dbx.files_upload(f.read(), dest_path)

else:

    upload_session_start_result = dbx.files_upload_session_start(f.read(CHUNK_SIZE))
    cursor = dropbox.files.UploadSessionCursor(session_id=upload_session_start_result.session_id,
                                               offset=f.tell())
    commit = dropbox.files.CommitInfo(path=dest_path)

    while f.tell() < file_size:
        if ((file_size - f.tell()) <= CHUNK_SIZE):
            print dbx.files_upload_session_finish(f.read(CHUNK_SIZE),
                                            cursor,
                                            commit)
        else:
            dbx.files_upload_session_append(f.read(CHUNK_SIZE),
                                            cursor.session_id,
                                            cursor.offset)
            cursor.offset = f.tell()

f.close()

but everytime i get the same exception when reaching 

dbx.files_upload_session_finish(f.read(CHUNK_SIZE),
                                            cursor,
                                            commit)
 

the exception is:

dropbox.exceptions.ApiError: ApiError('fa8b9a1a5c61684f4e6f58f2c695c248', UploadSessionFinishError('path', WriteError('malformed_path', None)))

I have been stuck on this for days, anyone has any suggestions? 

(I have tried already switching the following function to the v2 version, but it didn't helpped

files_upload_session_append