title: Why does my Firebase request return as 500 (internal server error)?
body: <p>I have the following code that does not execute (I get back 500 internal server error).</p>
  
  <p>What did I do wrong?</p>
  
  <pre><code>app.post('/api/sign-in', function (req, res, next) {
      //auth
      firebase.auth().signInWithEmailAndPassword(req.query.username, req.query.password)
          //get back promise, log user object's email address
          .then(function (user) {
              console.log(user.email)
          })
          //log error
          .catch(function (error) {
              console.log(error);
          })
  });
  </code></pre>