Bash testing -n

I thought the -z option in a bash test would be true if the condition evaluates to null and -n would be true if the condition is not null. So then why is this happening?

var1=
echo ${#var1} # 0

[ -n $var1 ] && echo 'hi' # hi
unset var1
[ -n $var1 ] && echo 'hi' # still prints hi!