A user lost time on their mocha test suite using chai matchers, as they where missing an await on an expect. The error shown was:

HardhatError: HHE100: An internal invariant was violated

We should try and detect for this case and show a clear error message suggesting that the likely cause is a lack of an await.

Example

it('Should transfer money to another wallet with extra value', async function () {
    const { accounts: { alice }, contracts: { drop }, others: { hashedElements, leaves, proofs, signature } } = await loadFixture(deployContractsFixture);
    const txn = await drop.claim(alice, 1, proofs[leaves.indexOf(hashedElements[0])], signature, { value: 10 });
    
    expect(txn).to.changeEtherBalance(ethers, alice, 10); // <-- missing await expect(txn)
});

Investigation

An initial look suggests something like the following is happening

1. By missing out the await on expect(txn).to.changeEtherBalance(ethers, alice, 10);, the test completes and passes
2. The test assertion is then run (in this case changeEtherBalance) because it was not awaited
3. The assertion uses ethers underlying provider to make a getBlock request. The provider can't connect and ethers returns null
4. The null block number trips the internal variant exception.