title: Can't create Braintree client token with customer ID
body: <p>Copied directly from Braintree's tutorial, you can create a client token with a customer ID like this:</p>
  
  <pre><code>gateway.clientToken.generate({
      customerId: aCustomerId
  }, function (err, response) {
      clientToken = response.clientToken
  });
  </code></pre>
  
  <p>I declare <code>var aCustomerId = "customer"</code> but node.js closes with the error </p>
  
  <pre><code>new TypeError('first argument must be a string or Buffer')
  </code></pre>
  
  <p>When I try to generate a token without the customerId, everything works fine (though I never get a new client token but that's another question). </p>
  
  <p>EDIT: Here is the complete test code as requested:</p>
  
  <pre><code>var http = require('http'),
      url=require('url'),
      fs=require('fs'),
      braintree=require('braintree');
  
  var clientToken;
  var gateway = braintree.connect({
      environment: braintree.Environment.Sandbox,
      merchantId: "xxx", //Real ID and Keys removed
      publicKey: "xxx",
      privateKey: "xxx"
  });
  
  gateway.clientToken.generate({
      customerId: "aCustomerId" //I've tried declaring this outside this block
  }, function (err, response) {
      clientToken = response.clientToken
  });
  
  http.createServer(function(req,res){
     res.writeHead(200, {'Content-Type': 'text/html'});
     res.write(clientToken);
     res.end("&lt;p&gt;This is the end&lt;/p&gt;");
  }).listen(8000, '127.0.0.1');
  </code></pre>