async TypeError: cb is not a function

I've created the following method to aid me in finding an item and updating another object attribute with certain value like this:

  mergeAinBbyAttr: function(a, b, attr, val, cb) {
    async.forEach(a, function(itemA, cb) {
        async.forEach(b, function(itemB, cb) {
            if (itemA[attr] == itemB[attr]) {
              itemB[val] = itemA[val];
            }
            cb(null, itemB);
          },
          function(e, result) {
            cb(e, result);
          });
      },
      function(e, result) {
        cb(e, result);
      });
  }

The problem is that I get:

TypeError: cb is not a function

in the last cb(e, result);. I have to admit I am a bit brain dead an the answer must be dead simple.

Thanks!

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