for loop question

[Ray Andrews <rayandrews@xxxxxxxxxxx>]

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?