title: Error in rpy2- No (non-missing) observations
body: <p>I m working with rpy2 on Ubuntu 1104. I m following the <a href="http://cran.r-project.org/doc/contrib/Fox-Companion/appendix-cox-regression.pdf" rel="nofollow">COX</a> file. The steps given in file are in R. I have to do same steps in python using rpy2. 
  I didn't get any tutorial with rpy2. I managed to write following,</p>
  
  <pre><code>from rpy2.robjects.packages import importr
  from rpy2.robjects import IntVector, Formula
  import rpy2.robjects as ro
  cox = importr("survival")
  csv = ro.vectors.DataFrame.from_csvfile('Rossi.txt', header=True, sep=' ')
  fmla = Formula('Surv(week, arrest) ~ fin + age + race + wexp + mar + paro + prio')
  mod_aalison = cox.coxph (fmla, data=csv)
  </code></pre>
  
  <p>But I'm getting following error,</p>
  
  <pre><code>&gt;&gt;&gt; mod_aalison = cox.coxph (fmla, data=csv)
  Error in function (formula, data, weights, subset, na.action, init, control,  : 
    No (non-missing) observations
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/usr/lib/python2.7/dist-packages/rpy2/robjects/functions.py", line 83, in __call__
      return super(SignatureTranslatedFunction, self).__call__(*args, **kwargs)
    File "/usr/lib/python2.7/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, weights, subset, na.action, init, control,  : 
    No (non-missing) observations
  </code></pre>
  
  <p>Am I missing any thing? I have no prior experience on R. 
  I'm not sure the if data in function coxph has to be in dataframe format.
  Any help is appreciated.</p>
  
  <p>This is link to <a href="http://cran.r-project.org/doc/contrib/Fox-Companion/Rossi.txt" rel="nofollow">Rossi.txt</a></p>