title: Error with nlme
body: <p>For <code>IGF</code> data from <code>nlme</code> library, I'm getting this error message:</p>
  
  <pre><code>lme(conc ~ 1, data=IGF, random=~age|Lot)
  Error in lme.formula(conc ~ 1, data = IGF, random = ~age | Lot) : 
    nlminb problem, convergence error code = 1
    message = iteration limit reached without convergence (10)
  </code></pre>
  
  <p>But everything is fine with this code</p>
  
  <pre><code>lme(conc ~ age, data=IGF)
  Linear mixed-effects model fit by REML
    Data: IGF 
    Log-restricted-likelihood: -297.1831
    Fixed: conc ~ age 
   (Intercept)          age 
   5.374974367 -0.002535021 
  
  Random effects:
   Formula: ~age | Lot
   Structure: General positive-definite
              StdDev      Corr  
  (Intercept) 0.082512196 (Intr)
  age         0.008092173 -1    
  Residual    0.820627711       
  
  Number of Observations: 237
  Number of Groups: 10 
  </code></pre>
  
  <p>As <code>IGF</code> is <code>groupedData</code>, so both codes are identical. I'm confused why the first code produces error. Thanks for your time and help.</p>