Ответ Google Text-To-Speech API 400 Ошибка (неверный аргумент), но ввод - строка

Question

Я немного растерялся. Я получаю ошибку 400 (неверный тип) из Google TTS-API.

Что я пытаюсь сделать: извлечь текст из PDF и использовать выходную строку в качестве входных данных для TTS. Тип txt = convert_pdf_to_txt ("test.pdf") является строкой. Так в чем же проблема? Если я изменю ввод на любую другую строку, она будет работать. Вот код:

from google.cloud import texttospeech
from google.oauth2 import service_account
import os
import io
from pdfminer.pdfinterp import PDFResourceManager, PDFPageInterpreter
from pdfminer.converter import TextConverter
from pdfminer.layout import LAParams
from pdfminer.pdfpage import PDFPage


def convert_pdf_to_txt(path):
    rsrcmgr = PDFResourceManager()
    retstr = io.StringIO()
    codec = 'utf-8'
    laparams = LAParams()
    device = TextConverter(rsrcmgr, retstr, codec=codec, laparams=laparams)
    fp = open(path, 'rb')
    interpreter = PDFPageInterpreter(rsrcmgr, device)
    password = ""
    maxpages = 0
    caching = True
    pagenos = set()

    for page in PDFPage.get_pages(fp, pagenos, maxpages=maxpages,
                                  password=password,
                                  caching=caching,
                                  check_extractable=True):
        interpreter.process_page(page)

    fp.close()
    device.close()
    text = retstr.getvalue()
    retstr.close()
    return text

txt = convert_pdf_to_txt("test.pdf")

# Language settings
# de-DE, en-US
language = "de-DE"
# de-DE-Wavenet-B, en-US-Wavenet-D
language_model = "de-DE-Wavenet-B"

# Credentials for Google's API
credentials_path = os.path.dirname(os.path.realpath(__file__)) + os.sep + 'credentials.json'
credentials = service_account.Credentials.from_service_account_file(credentials_path)

client = texttospeech.TextToSpeechClient(credentials=credentials)

# Set the text input to be synthesized
synthesis_input = texttospeech.types.SynthesisInput(text=txt)

voice = texttospeech.types.VoiceSelectionParams(
    language_code=language,
    name=language_model,
    ssml_gender=texttospeech.enums.SsmlVoiceGender.NEUTRAL)

audio_config = texttospeech.types.AudioConfig(
    audio_encoding=texttospeech.enums.AudioEncoding.MP3)

# Perform the text-to-speech request on the text input with the selected
# voice parameters and audio file type
response = client.synthesize_speech(synthesis_input, voice, audio_config)

# The response's audio_content is binary.
with open('output.mp3', 'wb') as out:
    # Write the response to the output file.
    out.write(response.audio_content)
    print('Audio content written to file "output.mp3"')

Будет ли вывод:

google.api_core.exceptions.InvalidArgument: 400 Request contains an invalid argument.