RPy2 robjects.r.pie gives curious error “'x' values must be positive”

Question

I'm feeling awfully silly, but I can't seem to work this out. I'm trying to make a pie chart, using rpy2 in Python.

from rpy2.robjects import r
import os.path

image = "test.png"
values = [0.5, 0.5]

print "using R"
r.png(image, width=100, height=100)
r.pie(values)
r.dev_off()

Now, to do the same thing directly in R I know I want this:

values <- (0.5, 0.5)
pie(values)

That works fine in the R interpreter. I've tried using tuples instead of lists in Python, but was told ValueError: Nothing can be done for the type <type 'tuple'> at the moment.

What Python type corresponds to the R vector? Do I need to use numpy?