[Orgmode] Bug - using Plain lists and Checkboxes in the same level

[iemacs]

The file content is something like below example
#+BEGIN_BLOCKQUOTE
1. a

- [ ] b
#+END_BLOCKQUOTE

If I press Alt-Enter at the end of first line, I get
#+BEGIN_BLOCKQUOTE
1. a
2.

3. [ ] b
#+END_BLOCKQUOTE

It seems that plain lists can not be mixed with checkboxes in the same level.

The org-version is 6.13a.

-- 
Kind regards,

Tian Qiu