ModelScope下面提供的例子运行后怎么第二个参数null报错？

ModelScope下面提供的例子运行后怎么第二个参数null报错？
from gradio_client import Client

client = Client("https://modelscope.cn/api/v1/studio/Fengshenbang/Ziya_LLaMA_13B_v1_online/gradio/")
result = client.predict(
"Howdy!", # str representing input in 'Input' Textbox component
"null", # str representing input in 'Ziya' Chatbot component
fn_index=0
)
print(result)