title: error using statsmodels OLS: returning nan values
body: <p>I have a data set like:</p>
  
  <pre><code>Growth  NHSPSTY% Index  USURTOT Index   GLPFTOCI Index  CPTICHNG Index  NAPMPMI Index   RSTAXYOY Index  SAARTOTL Index  USASHVTK Index  CONCCONF Index  LEI TOTL Index  SPX Index   TOT_DEBT_TO_TOT_EQY BDIY Index  cry index   CO1 Comdty
  Date                                                                
  1998-03-31  4.1 7.5 4.7 0.121000    83.5325 52.9    2.9 -0.032258   0.404   133.80  88.9    0.455185    197.26  966 169.04  14.26
  1998-06-30  3.8 9.8 4.5 0.125556    82.2970 48.9    4.5 0.154930    0.393   138.23  88.6    0.280973    204.65  856 152.58  13.38
  </code></pre>
  
  <p>I wanted to run a OLS regression but all the parameters returned all nan values.
  And it warned:</p>
  
  <pre><code>/Users/jake/anaconda3/lib/python3.6/site-packages/scipy/stats/_distn_infrastructure.py:879: RuntimeWarning: invalid value encountered in greater
    return (self.a &lt; x) &amp; (x &lt; self.b)
  /Users/jake/anaconda3/lib/python3.6/site-packages/scipy/stats/_distn_infrastructure.py:879: RuntimeWarning: invalid value encountered in less
    return (self.a &lt; x) &amp; (x &lt; self.b)
  /Users/jake/anaconda3/lib/python3.6/site-packages/scipy/stats/_distn_infrastructure.py:1821: RuntimeWarning: invalid value encountered in less_equal
    cond2 = cond0 &amp; (x &lt;= self.a)
  
  
  coef    std err t   P&gt;|t|   [0.025  0.975]
  const   nan nan nan nan nan nan
  NHSPSTY% Index  nan nan nan nan nan nan
  USURTOT Index   nan nan nan nan nan nan
  GLPFTOCI Index  nan nan nan nan nan nan
  CPTICHNG Index  nan nan nan nan nan nan
  </code></pre>
  
  <p>My command:</p>
  
  <pre><code>import statsmodels.api as sm
  model = sm.OLS(data.Growth,sm.add_constant(data.iloc[:,1:])).fit()
  model.summary()
  </code></pre>