Emacs Orgmode table $> References does not work - emacs

Question

GNU Emacs 24.4.1 org-mode
Here is an org-mode table
#+TBLNAME: revenue
| / | < | | < | | < | | | | | | | | | | | |
| Product | Year_SUM | Month_SUM | Platform | Platform_SUM | adwo | AdMob | adChina | adSage | appfigures | appdriver | coco | Domob | Dianru | Limei | guohead | youmi |
| | | | | | | | | | | | | | | | | |
|---------+----------+-----------+----------+------------------+------+-------+---------+--------+------------+-----------+------+-------+--------+-------+---------+-------|
| Jan | | | iOS | #ERROR | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| | | | Android | =vsum($6..$>);NE | | 1 | | 1 | | 1 | | 1 | | 1 | | 1 |
|---------+----------+-----------+----------+------------------+------+-------+---------+--------+------------+-----------+------+-------+--------+-------+---------+-------|
| | | | | | | | | | | | | | | | | |
#+TBLFM: $5=vsum($6..$>);NE
As you see ,the formula $5=vsum($6..$>);NE can't be calculated! Here is debug info:
Substitution history of formula
Orig: vsum($6..$>)
$xyz-> vsum($6..$>)
#r$c-> vsum($6..$>)
$1-> vsum((0)..$>)
--------^
Error: Expected `)'
But if I replace the formula with $5=vsum($6..$17) and then it works ,I can't figure out where is the problem?
I need some help ,appreciate it!