无法反序列化关键数据

我正在使用pyjwt库生成JWT令牌。编码时出现此错误。我将用于编码的代码放在下面。

码：

import datetime
import jwt

PROJECT_ID = 'test-iot-core-278019'
SSL_PRIVATE_KEY_FILEPATH = 'roots.pem'
SSL_ALGORITHM = 'ES256'

def create_jwt(project_id, private_key_file, algorithm):
    token = {
        # The time that the token was issued at
        'iat': datetime.datetime.utcnow(),
        # The time the token expires.
        'exp': datetime.datetime.utcnow() + datetime.timedelta(minutes=60),
        # The audience field should always be set to the GCP project id.
        'aud': project_id
    }

    # Read the private key file.
    with open(private_key_file, 'r') as f:
        private_key = f.read()

    print('Creating JWT using {} from private key file {}'.format(algorithm, private_key_file))

    return jwt.encode(token, private_key, algorithm=algorithm)


print('JWT token:')
print(create_jwt(PROJECT_ID, SSL_PRIVATE_KEY_FILEPATH, SSL_ALGORITHM))

错误：

ValueError: Could not deserialize key data.