nls question

I'm running into problems trying to use the nls function to fit the some
data.  I'm invoking nls using 

nls(s~k/(a+r)^b, start=list(k=1, a=13, b=0.59))

but I get errors indicating that the step has been reduced below the
minimum step size or an inifinity is generated in numericDeriv. I've
tried to use a variety of starting values for a, b, k but get similar
errors.  

Is there anything I can do to get the a fit or is there an alternative
to the nls function?

Suchandra Thapa <s-thapa-11 at alumni.uchicago.edu>