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

This is my hardhat.config.js -

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.