I am using Open Zeppelin's Test Helpers in my Hardhat testing to check for events emitted using the librarys expectEvent. The code I have has worked fine for other tests. The code would look like the following:

const reciept= await marketplace.listNFT(
  { from: USER, value: ethers.utils.parseUnits("1","ether")}
expectEvent(reciept, 'Listed', { 
   owner: TOKEN_OWNER,
   user: USER,
   nftContract: nft.address.toString(),
   tokenId: tokenId.toString(),
   startDateUNIX: new BN(TOMORROW),
   endDateUNIX: new BN(IN_FIVE_DAYS),
   expires: new BN(TODAY_2),
   Fee: new BN(0)

The event emits properly and I am able to test and verify other events except for this test case. I am having trouble with Fee. Initially I put 0 I get the following error:

 expected event argument 'Fee' to have value 0 but got 1000000000000000000
      + expected - actual


This is fine as I was creating the unit test for the first time and made a typo. Instead of 0 it should be 1000000000000000000 as I expect the fee 1Eth. So I changed the line to correctly reflect the output like below so it would pass:

  Fee: new BN(1000000000000000000)

I reran the test expecting this to pass. Instead I arrive at a new vague error stating assertion failed. Im unsure why its failing. Is this because of an overflow? How can I test the fee? I dont understand why the error is occuring

Error: Assertion failed
      at assert (node_modules/bn.js/lib/bn.js:6:21)
      at BN._initNumber (node_modules/bn.js/lib/bn.js:128:7)
      at BN.init [as _init] (node_modules/bn.js/lib/bn.js:82:19)
      at new BN (node_modules/bn.js/lib/bn.js:39:12)
      at new BNwrapped (node_modules/web3-utils/lib/utils.js:480:16)
      at Context.<anonymous> (test/Marketplace.js:447:14)
      at processTicksAndRejections (node:internal/process/task_queues:95:5)

1 Answer 1


For big integer you should construct the value from string instead of literal value because uint32 or uint64 native types cannot handle that. The below should work:

Fee: new BN('1000000000000000000')
  • Ah! Thank you. I have other constant variables defined like TOMORROW IN_FIVE_DAYS which are big numbers too. ideally should I be converting them to string ? TOMORROW.toString()
    – Potion
    Commented Dec 23, 2022 at 6:46
  • I'm not sure what that constant means, if it means the UNIX timestamp then it may not be necessary. Otherwise, the safest choice should be using string for BN literals
    – minhhn2910
    Commented Dec 23, 2022 at 6:48
  • Good point. Yes theyre unix timestamps represented in seconds so its not necessary. Thank you. Answer accepted.
    – Potion
    Commented Dec 23, 2022 at 6:53

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