su to root leaves me with the users history in zsh

[Angelina Carlton <brat@xxxxxxxx>]

Debian unstable
zsh 4.3.2-6
login 1:4.0.15-4

If I type, as a user, su and login with the root password, then with either
up arrow or ctrl-r the command history is 3976 items (max set to 5000)
but they are the *users* commands, not roots. There are some of roots
commands interspersed but those only seem to be from the last root session.

su - gives me the correct 4819 commands that root has issued since the
machine was installed. 

I thought something was wrong with my system when I first noticed this
but it also occurs on my Debian sarge server. I am not having much luck
with google so I am wondering if this is normal behaviour for zsh and
if so, how can I "fix" it?

-- 
-----Angelina Carlton-----
orchid on irc.freenode.net
     brat@xxxxxxxx
web:bzgirl.bakadigital.com
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