NounsAuctionHouse.sol

// ...

function _createAuction() internal {
    uint256 nounId;

    // Check if the auction contract itself has any Noun tokens
    uint256 auctionContractNounCount = nouns.balanceOf(address(this));
    if (auctionContractNounCount > 0) {
        // Use the first Noun token found in the auction contract
        nounId = nouns.tokenOfOwnerByIndex(address(this), 0);
    } else {
        // Mint a new Noun token if the auction contract has none
        try nouns.mint() returns (uint256 _nounId) {
            nounId = _nounId;
        } catch Error(string memory) {
            _pause();
            return;
        }
    }

    uint256 startTime = block.timestamp;
    uint256 endTime = startTime + duration;

    auction = Auction({
        nounId: nounId,
        amount: 0,
        startTime: startTime,
        endTime: endTime,
        bidder: payable(0),
        settled: false
    });

    emit AuctionCreated(nounId, startTime, endTime);
}

// ...