[Unexpected token R in JSON at position 0] when access another workflow

I have 2 very simple workflows: Hello and Access Hello.

In Access Hello workflow I used a API call node to access Hello workflow with its Endpoint URL.
when I test Hello, it works very well. However, when I test Access Hello, it always prompt:
Unexpected token R in JSON at position 0.
Please find the attachment [image]

Why is that? What shoul I do now?

Thanks a lot.