H88 Error: Invalid account: #0 for network: mumbai – Expected string, received undefined

This is my hardhat.config.js file code

module.exports = {
  solidity: "0.8.4",
  networks: {
    hardhat: {
      chainId: 1337
    },
    mumbai: {
      url: "https://rpc-mumbai.matic.today",
      accounts: [process.env.pk]
    },
    // polygon: {
    //   url: "https://polygon-rpc.com/",
    //   accounts: [process.env.pk]
    // }
  }
};

When running npx hardhat test the following error appears:

**Error HH8: There’s one or more errors in your config file:

• Invalid account: #0 for network: mumbai – Expected string, received undefined**

Seems I have a couple of errors with my hardhat.config.js file but can’t locate. I am using Nader Dabit’s full-stack-web3 tutorial for full stack web3 development.