bitcoind – Permission denied when attempting to backup wallet using backupwallet

When I call bitcoin@raspberrypi:~ $ bitcoin-cli backupwallet "backup.dat", I get the following error:

error code: -4

error message:

Error: Wallet backup failed!

~/.bitcoin/debug.log shows the following:

error copying wallet.dat to backup.dat – boost::filesystem::copy_file: Permission denied: “/home/bitcoin/.bitcoin/new_wallet/wallet.dat”, “backup.dat”

Is it something I’m doing wrong? Do I need to give permissions to user “bitcoin”?

The wallet is not encrypted.

Thanks