I'm playing Ethernaut (https://ethernaut.openzeppelin.com/).

I'm currently on level 3 (Coin Flip).

This is the Coin Flip contract:

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import '@openzeppelin/contracts/math/SafeMath.sol';

contract CoinFlip {

  using SafeMath for uint256;
  uint256 public consecutiveWins;
  uint256 lastHash;
  uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;

  constructor() public {
    consecutiveWins = 0;

  function flip(bool _guess) public returns (bool) {
    uint256 blockValue = uint256(blockhash(block.number.sub(1)));

    if (lastHash == blockValue) {

    lastHash = blockValue;
    uint256 coinFlip = blockValue.div(FACTOR);
    bool side = coinFlip == 1 ? true : false;

    if (side == _guess) {
      return true;
    } else {
      consecutiveWins = 0;
      return false;

I deployed this code in the Remix IDE.

However, whenever I call flip(), I get:

transact to CoinFlip.flip errored: NotFoundError: Key not found in database [h n]

I searched for this problem online, but most related questions involve Web3, Ganache, RPCs or something of the sort. And the answers aren't really what I'm looking for.

I tried:

  • flip()
  • flip(0)
  • flip(1)
  • flip(false)
  • flip(true)
  • flip("false")
  • flip("true")
  • increasing the gasLimit
  • refreshing the page

Nothing fixed it. Same error gets thrown.

Am I doing something wrong? Is the problem in the Smart Contract? Or is it in Remix?

Secondary Question: What's the proper way to pass in a bool parameter to a function?

0 and 1? false and true? "false" and "true"?


1 Answer 1


I encountered the same error when solving the same Ethernaut level. I'm using Remix IDE. When I changed from deploying with JavaScript VM to deploying with Injected Web3 (there's a dropdown menu in DEPLOY & RUN TRANSACTIONS in Remix and you need MetaMask to deploy this way), the error disappeared.

I added the following function to the contract:

  function blockValue() view public returns (uint256) {
      return uint256(blockhash(block.number - 1));

and it was still throwing the error when calling blockValue instead of flip. So I think the reason is that the simulation of blocks in JavaScript VM isn't realistic enough for blockhash to work properly.

Answer to secondary question: I passed true and it incremented consecutiveWins, so true and false should be correct.

  • Encountered and figured the same: blockhash(block.number - 1) seems to be the issue, probably because there is no previous block in a fresh network.
    – gadget
    Nov 17, 2021 at 12:00
  • Yes, blockhash() does not work properly on the remix testnet
    – Lynn Han
    Mar 7, 2022 at 8:14

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