for loop question

[Ray Andrews <rayandrews@eastlink.ca>]

test()
{

     # THIS WORKS  FINE:

     i=1
     while [ -n "$TLC[i]" ]; do
       let i=i+1
       print -ru2 $TLC[i]
     done

     # THIS WORKS FINE:

     for ((i=1; i<6; i++))
     do
       print -ru2 $TLC[i]
     done

     # ... BUT THIS FAILS WITH "BAD OUTPUT FORMAT SPECIFICATION":

     for ((i=1; [ -n "$TLC[i]" ]; i++))
     do
       print -ru2 $TLC[i]
     done
}

I've fiddled around with various modifications but it seems to dislike " 
[ -n "$TLC[i]" ] "
even though the 'while' loop is happy with it.  That expression 
evaluates to true or
false, does it not? So why won't 'for' swallow it?