miner.stop test, but it happen error.

miner_stop_test.js

it('multi transfer in same block', async () => {
  web3.extend({property: 'mine', methods: [{ name: 'start', call: 'miner_start', params:0}]})
  web3.extend({property: 'mine', methods: [{ name: 'stop', call: 'miner_stop', params:0}]})

  const instance = await MyERC20Token.new()
  const beforeBlock = await web3.eth.getBlockNumber()

  await web3.mine.stop()
  
  // queued tx1
  await instance.transfer(accounts[1], 100)
  // queued tx2
  await instance.transfer(accounts[2], 100)

  await web3.mine.start()
  const afterBlock = await web3.eth.getBlockNumber();

  assert.equal(beforeBlock+1, afterBlock) // block increase just +1.
  assert.equal(await Instance.balanceOf(accounts[1]), 100)
  assert.equal(await Instance.balanceOf(accounts[2]), 100)
})