Dumping collections does not work with marshmallow 3.5.1 / Flask 1.1.1

$ cat test.py 

from flask import Flask
from flask_marshmallow import Marshmallow
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)

db = SQLAlchemy(app)
ma = Marshmallow(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String)

class UserSchema(ma.Schema):
    class Meta:
        fields = ("id", "name")

user_schema = UserSchema()
users_schema = UserSchema(many=True)

@app.route("/api/users/")
def users():
    all_users = User.query.all()
    return users_schema.dump(all_users)


@app.route("/api/users/<id>")
def user_detail(id):
    user = User.query.get(id)
    return user_schema.dump(user)

dump(user) works fine

$ curl localhost:5000/api/users/1

{
  "id": 1, 
  "name": "John"
}

dump(all_users) fails:

$ curl -L localhost:5000/api/user

[...]
Traceback (most recent call last):
  File "venv/lib/python3.7/site-packages/flask/app.py", line 2463, in __call__
    return self.wsgi_app(environ, start_response)
  File "venv/lib/python3.7/site-packages/flask/app.py", line 2449, in wsgi_app
    response = self.handle_exception(e)
  File "venv/lib/python3.7/site-packages/flask/app.py", line 1866, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "venv/lib/python3.7/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "venv/lib/python3.7/site-packages/flask/app.py", line 2446, in wsgi_app
    response = self.full_dispatch_request()
  File "venv/lib/python3.7/site-packages/flask/app.py", line 1952, in full_dispatch_request
    return self.finalize_request(rv)
  File "venv/lib/python3.7/site-packages/flask/app.py", line 1967, in finalize_request
    response = self.make_response(rv)
  File "venv/lib/python3.7/site-packages/flask/app.py", line 2130, in make_response
    " {rv.__class__.__name__}.".format(rv=rv)
TypeError: The view function did not return a valid response. The return type must be a string, dict, tuple, Response instance, or WSGI callable, but it was a list.

[jimmy-lt]

@deckar01 Indeed, the bug I’m talking about was caused by my own code.

[deckar01]

@spack971 That appears to be unrelated. Can you open a separate issue?