zoo plot without box?

Hi

I'm trying to create a zoo plot without the box surrounding it. If it 
was a simple data, I woudl do:
plot(1:100, bty="n")

but with zoo, the argument bty does not seem to work?
library(zoo)
plot(zoo(1:100), bty="n")

I thought nervertheless that par arguments are allowed in plot.zoo... 
Any idea about a workaround?

Thanks!!

Matthieu