title: Calling web3.utils.BN.add gives error: Cannot create property 'negative' on number
body: <p>At truffle test, I am trying to add two Big Numbers together using the following code:</p>
  
  <pre><code>myBignumber = new web3.utils.BN(1);
  myBignumber.add(2)
  </code></pre>
  
  <p>But it caused this error:</p>
  
  <blockquote>
    <p>Cannot create property 'negative' on number 2</p>
  </blockquote>
  
  <p>Versions:</p>
  
  <ul>
  <li>Truffle 5.0 </li>
  <li>Web3 1.0</li>
  </ul>