title: async - TypeError: Cannot read property 'apply' of undefined
body: <p>I have this simple code which seems to be aligned with how async.waterfall should be used and yet I have this TypeError mentionned:</p>
  
  <pre><code>var myCallback = function (data){
    console.log("task done for " + data);
  }
  
  var test = function (param,callback) {
    console.log("in test, doing some work, param is: " + param);
    callback(param);
  };
  
  async.waterfall([
    test("call 1",myCallback),
    test("call 2",myCallback),
    test("call 3",myCallback)
  ],  function (err, result) {
      // result now equals 'done'
  });
  </code></pre>
  
  <p>The result of the execution is:</p>
  
  <pre><code>in test, doing some work, param is: call 1
  task done for call 1
  in test, doing some work, param is: call 2
  task done for call 2
  in test, doing some work, param is: call 3
  task done for call 3
  /Users/.../Documents/.../node_modules/async/dist/async.js:5021
              task.apply(null, args);
                  ^
  
  TypeError: Cannot read property 'apply' of undefined
      at nextTask (/.../node_modules/async/dist/async.js:5021:17)
      at Object.waterfall (/.../node_modules/async/dist/async.js:5024:9)
      at Object.&lt;anonymous&gt; (/.../app.js:77:7)
      at Module._compile (module.js:409:26)
      at Object.Module._extensions..js (module.js:416:10)
      at Module.load (module.js:343:32)
      at Function.Module._load (module.js:300:12)
      at Function.Module.runMain (module.js:441:10)
      at startup (node.js:139:18)
      at node.js:968:3
  </code></pre>
  
  <p>I've been looking around but could not find the issue - some help welcomed. Many thanks! C</p>