rpy2 problems, nls passing list() as argument from python to R

I am trying to fit a nonlinear curve using rpy2 from numpy array, but are stuck as I do not know how to pass the 'start' argument on the R side. I use R 2.12.1 and python 2.6.6

Error in function (formula, data = parent.frame(), start, control = nls.control(),  : 
parameters without starting value in 'data': responsev, predictorv
Traceback (most recent call last):
File "./employmentsHoro.py", line 279, in <module>
nls.nls2(formula=formula, data=dataf, start=mylist)
File "/usr/lib/python2.6/dist-packages/rpy2/robjects/functions.py", line 83, in __call__
return super(SignatureTranslatedFunction, self).__c开发者_如何学Call__(*args, **kwargs)
File "/usr/lib/python2.6/dist-packages/rpy2/robjects/functions.py", line 35, in __call__
res = super(Function, self).__call__(*new_args, **new_kwargs)
rpy2.rinterface.RRuntimeError: Error in function (formula, data = parent.frame(),start, control = nls.control(),  : 
parameters without starting value in 'data': responsev, predictorv

Can anyone help me determine how to pass a list() object to the nls formula?

the relevant part of my code is this:

import rpy2.robjects as robjects
from rpy2.robjects import DataFrame, Formula
import rpy2.robjects.numpy2ri as npr
import numpy as np
from rpy2.robjects.packages import importr
nls = importr('nls2')
stats = importr('stats')

mylist = robjects.r('list(a=700,b=0.8,c=200000)')

dataf = DataFrame({'responsev': professions, 'predictorv': totalEmployment})
starter= DataFrame({'a':700,'b':0.80,'c':200000})
formula = Formula('responsev ~I( a*(predictorv/c)^b )/( 1+( predictorv/c )^b )')
nls.nls2(formula=formula, data=dataf, start=starter)

The main error is this one:

Error in function (formula, data = parent.frame(), start, control = 
nls.control(),  : parameters without starting value in 
    'data': responsev, predictorv

Where are declared the variable professions? and DataEmployment? seems they don't have a starting value, maybe you have to change/transform in something that R understands?