when testing my hardhat configjs i find this problem what's the problem and how to resolve it please?

:\Users\DELL\nft_marketplace>npx hardhat test

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

• Invalid account: #0 for network: mumbai - Expected string, received function
• Invalid account: #0 for network: mainnet - Expected string, received function

To learn more about Hardhat's configuration, please go to https://hardhat.org/config/

For more info go to https://hardhat.org/HH8 or run Hardhat with --show-stack-traces