WatsonException: Error: unsupported media type, Code: 415

from watson_developer_cloud import AlchemyLanguageV1
alchemy_language = AlchemyLanguageV1(url="https://gateway.watsonplatform.net/natural-language-understanding/api",
          username="xxx",
          password="xxx")
print(json.dumps(alchemy_language.sentiment(text="hehe")))