I encounted this error using version 4.  I thought I was getting this error in version 3 b/c the variable n only appears as period (t+1), but the requirement that all variables show up in the current period as well has been dropped in version 4, so now I am not so sure.   
 
  
.   Any ideas?
Thanks a lot.  My code is attached.
You do not have the required permissions to view the files attached to this post.