Error for docker compose in the following yaml file?

I've this compose file but when I try to execute it an error is thrown:

ERROR: The Compose file '.\docker-compose.yml' is invalid because: services.db.environment contains an invalid type, it should be an object, or an array.

version: '3.1'

services:
  my-apache2:
    build:
      context: .
      dockerfile: Dockerfile
    ports:
      - 80:80
    volumes:
      - ./src:/var/www/html/

  db:
    image: mysql
    command: --default-authentication-plugin=mysql_native_password
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD:oh686bf

  adminer:
    image: adminer
    restart: always
    ports:
      - 8080:8080 

The above yaml file is for entering data from a HTML file to the MySQL server.
Can anybody tell me what is wrong in environment in this yml file?