Can't get static files work in version 0.6.1

Example:

from apiflask import APIFlask
app = APIFlask(__name__)

@app.route("/")
def index():
    return ""

h1 { font-size: 16px; }

File Tree:

/app-root
  app.py
  /static
    style.css

And I GET /static/style.css in the browser, I expected to see:

h1 { font-size: 16px; }

But I got

500 Internal Server Error

What I have tried

1. Change APIFlask to Flask, it works well.
2. Change to any older version of APIFlask, it doesn't work.

Environment:

• Python version: 3.8.5
• Flask version: 2.0.0
• APIFlask version: 0.6.1