title: Strange error with Dredge: MuMIn
body: <p>I am wondering whether anyone could help me trouble shoot this error from a task with <em>dredge</em> package:</p>
  
  <pre><code>   Fixed term is "(Intercept)"
   |============================================================================                                                          
   63%
  Error in while ((iComb &lt;- iComb + 1L) &lt; ncomb) { : 
  missing value where TRUE/FALSE needed
  In addition: Warning message:
  In iComb + 1L : NAs produced by integer overflow
  
  Timing stopped at: 90799.82 18.545 90796.9 
  &gt; 
  </code></pre>
  
  <p>I see that it probably has to do with my TRUE/FALSE subset matrix but I have double checked to make sure the matrix contains all terms returned by calling getAllTerms function.</p>
  
  <p>Here is how I am calling <em>dredre</em></p>
  
  <pre><code>#exclude only pairs of variables having cor. coefficient r &gt; 0.2
  smat &lt;- abs(cor(predictors)) &lt;= .2
  smat[!lower.tri(smat)] &lt;- NA
  
  #set global model
  forml&lt;-glm(SEVERITY_CODE ~(autocorr+CummDHM_retro + CummDHM_sat +     DHM_retro_cnt + DHM_sat_cnt + diff_events_all + diff_events_periodmax + duration_events_neg + duration_events_pos + events_neg_all + events_neg_periodmax + events_pos_all + events_pos_periodmax + maxpower_events_neg + maxpower_events_neg_norm + maxpower_events_pos + maxpower_events_pos_norm + maxpower_neg + maxpower_neg_norm + maxpower_pos + maxpower_pos_norm + NSE + pct_coi_periodmax + pct_ispos_signif + pct_seasonal + pct_signif_periodmax + period_events_neg + period_events_pos + period_max + var_seasonal + varSST ), family=binomial(link=logit),  data=all.data)
  
  options(na.action = "na.fail") 
  
  system.time(modelmix &lt;- dredge(forml, subset = smat, trace=2, evaluate=FALSE,     extra = c("R^2", F = function(x)
  summary(x)$fstatistic[[1]]) ))
  </code></pre>