receiver not declared in useNTT contract in README

[thebraveman17]

README contains the useNTT contract:

contract UseNTT {
  function useNTT() public {
    string memory name = "Non-transferrable NFT";
    string memory symbol = "NTT";
    bool isLocked = true;

    NTT ntt = new NTT(name, symbol, isLocked);

    address minter = address(this);
    uint256 tokenId = 0;
    ntt.safeMint(minter, tokenId);

    ntt.transferFrom(minter, receiver, tokenId); // revert. token is locked.
  }
}

In the line ntt.transferFrom(minter, receiver, tokenId);, receiver is not declared.