运行docker-compose up后如何解决“ services.secrets的不受支持的配置选项”错误？

Question

运行以下命令后

sudo docker-compose up

我收到此错误消息

ERROR: The Compose file './docker-compose.yml' is invalid because:
Unsupported config option for services.secrets: 'db_password'

docker-compose.yml

version: '3.7'
services:
  web:
    build: .
    command: npm start
    volumes:
      - ./src:/usr/app/
      - ./src/node_modules:/usr/app/node_modules
    ports:
      - 80:8080
    environment:
      DB_NAME: "testdb"
      DB_USER: /run/secrets/db_user
      DB_PASSWORD: /run/secrets/db_password
    secrets:
      - db_user
      - db_password
  db:
    build: mongo:latest
    volumes:
      - mongo:/data/db/
      - ./src/node_modules:/usr/app/node_modules
    ports:
      - "27017:27017"
    environment:
      MONGO_INITDB_DATABASE: "websitedb"
      MONGO_INITDB_ROOT_USERNAME_FILE: /run/secrets/db_user
      MONGO_INITDB_ROOT_PASSWORD_FILE: /run/secrets/db_password
    secrets:
      - db_user
      - db_password
  secrets:
    db_user:
      file: ./db_user.txt
    db_password:
      file: ./db_pass.txt

这是我的 Docker版本：

Client:
 Version:           18.09.3
 API version:       1.39
 Go version:        go1.10.8
 OS/Arch:           linux/amd64
 Experimental:      false

我已经阅读了几篇文章，但无法解决。有人可以告诉我我在做什么错吗？