Redis MULTI transaction randomly returning null on EXEC callback in NodeJS

I have a problem where EXEC's callback randomly returns some values as nulls.

The code works fine most of the times, but it fails randomly (or if I refresh the browser repeatedly)...

Here's the code reduced to the point where it's failing:

var transaction = client.multi();
reply.forEach(function (id) { // reply always equals [ 'mykey1', 'mykey2' ]
  transaction.hgetall(namespace + ":" + id);
});
transaction.exec(function (err, replies) {
  // 'replies' sometimes returns all the responses properly,
  // other times it returns some of the values as null
  // See the examples I wrote below
});

When it works fine, the exec callback returns this:

[{
  owner: '123',
  id: 'asdasdasd',
  name: 'asdasdasd',
  created_at: '2012-10-06T09:26:25.596Z',
  updated_at: '2012-10-06T09:28:54.929Z'
},
{
  owner: '456',
  id: 'asdfsdfasdf',
  name: 'asdfsdfasdf',
  created_at: '2012-10-06T09:27:19.251Z',
  updated_at: '2012-10-06T09:28:03.116Z'
}]

When it doesn't work, it's returning this: (notice the null value)

[{
  owner: '123',
  id: 'asdasdasd',
  name: 'asdasdasd',
  created_at: '2012-10-06T09:26:25.596Z',
  updated_at: '2012-10-06T09:28:54.929Z'
}, null]