I really need your guy's help as I'm stuck in this for a week. Actually, I'm implementing a swap function on pancakeswap, I'm following this documentation. I'm doing swapEthForExactTokens but getting an error that I don't understand. I wrote the function...

//SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.11;

import "@theanthill/pancake-swap-periphery/contracts/interfaces/IPancakeRouter01.sol";
import "@theanthill/pancake-swap-periphery/contracts/interfaces/IERC20.sol";

contract Pancakeswap {
    address internal constant PANCAKESWAP_ROUTER_ADDRESS = 0x10ED43C718714eb63d5aA57B78B54704E256024E;
    address internal constant PANCAKE_FACTORY_ADDRESS = 0xcA143Ce32Fe78f1f7019d7d551a6402fC5350c73;

    address internal constant BTCB = 0x7130d2A12B9BCbFAe4f2634d864A1Ee1Ce3Ead9c;
    address private constant WETH = 0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2;
    address private constant WBNB = 0xbb4CdB9CBd36B01bD1cBaEBF2De08d9173bc095c;

    IPancakeRouter01 public pancakeswapRouter;

    constructor() {
        pancakeswapRouter = IPancakeRouter01(PANCAKESWAP_ROUTER_ADDRESS);

    function getPath(address arg0, address arg1) public pure returns (address[] memory) {
        address[] memory path = new address[](2);
        path[0] = arg0;
        path[1] = arg1;
        return path;

    function swappingETHForExactTokens(uint256 amountOut, address tokenOut) public payable returns (uint256[] memory) {
        uint256 deadline = block.timestamp + 15;
        uint256[] memory swappedAmounts = pancakeswapRouter.swapETHForExactTokens{
          value: msg.value}(amountOut, getPath(WETH, tokenOut), address(this), deadline);

        (bool success, ) = msg.sender.call{ value: address(this).balance }("");
        require(success, "refund failed");
        return swappedAmounts;

When it comes to unit testing, I'm doing it with typescript like the following.

export function shouldBehaveLikePancakeswap(): void {
  const AMOUNT_OUT = 10;
  const AMOUNT_IN = 100;
  const WHALE = process.env.WETH_WHALE;
  const DAI = process.env.DAI;
  const WBTC = process.env.WBTC;
  const TOKEN_OUT = DAI;
  const expectedSwapAmount = "557227237267357629";

  let TestPancakeswap;
  let pancakeswap;
  let receiver: { address: any };
  let other;

  it("should swap eth for exact tokens", async function () {
    [other, receiver] = await ethers.getSigners();
    TestPancakeswap = await ethers.getContractFactory("Pancakeswap");
    pancakeswap = await TestPancakeswap.deploy();

    const amounts = await pancakeswap.swappingETHForExactTokens(AMOUNT_OUT, TOKEN_OUT, {value: expectedSwapAmount});
    console.log(`amounts of token received ${amounts}`);

and in the last, calling shouldBehaveLikePancakeswap() from the main testing file. I'm running all this after forking mainnet on hardhat like below...


networks: {
    develop: {
      url: "",
      timeout: 200000,
      allowUnlimitedContractSize: true,
    hardhat: {
      forking: {
        url: "https://eth-mainnet.alchemyapi.io/v2/$MY_ALCHEMY_ID",
        blockNumber: 14389000,
      accounts: {
      chainId: chainIds.hardhat,

and then, with command: yarn test --network hardhat I'm running, and in return, getting this error...

1) Unit tests
         should swap eth for exact tokens:
     Error: Transaction reverted: function returned an unexpected amount of data
    at Pancakeswap.getEstimatedEthForTokens (contracts/Pancakeswap.sol:36)
    at Pancakeswap.swappingETHForExactTokens (contracts/Pancakeswap.sol:40)
    at async HardhatNode._mineBlockWithPendingTxs (node_modules/hardhat/src/internal/hardhat-network/provider/node.ts:1772:23)
    at async HardhatNode.mineBlock (node_modules/hardhat/src/internal/hardhat-network/provider/node.ts:466:16)
    at async EthModule._sendTransactionAndReturnHash (node_modules/hardhat/src/internal/hardhat-network/provider/modules/eth.ts:1496:18)
    at async HardhatNetworkProvider.request (node_modules/hardhat/src/internal/hardhat-network/provider/provider.ts:118:18)
    at async EthersProviderWrapper.send (node_modules/@nomiclabs/hardhat-ethers/src/internal/ethers-provider-wrapper.ts:13:20)

The first two lines of error says, I'm getting the error in lines...

uint256[] memory swappedAmounts = pancakeswapRouter.swapETHForExactTokens{
          value: ethAmountDeriveable}(amountOut, getPath(WETH, tokenOut), address(this), deadline);

1 Answer 1


In my program, the program works when I remove the 'blockNumber' statement in hardhat.config file. I'm not sure why.

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