replaceitem doesn't work

I tried to use the /replaceitem command, but it does not work.
I used the following command:
/replaceitem @e[x=991,y=20,z=1000,r=2] slot.armor.head skull 1 0
The target was an ArmorStand that was standing on 991,20,1001
That the syntax isn't working I saw on another bug report. It says "/replaceitem <entity|block>"