Can't preview drafts

I can't preview the posts with published set to false. What I did is described below:

1. set published: false in the yaml front matter of my post.
2. run rake preview.
3. go to localhost:4000, the post with published set to false didn't show up.

I search around and seems like rake preview should be able to preview the drafts, but I don't know why it wasn't working for me.