title: stepAIC error: AIC is -infinity
body: <p>I am using <code>stepAIC</code> in the MASS package for model selection and receive an error: “AIC is -infinity for this model, so 'step' cannot proceed” when including all candidate predictor variables.</p>
  
  <p>I am able to successfully execute <code>stepAIC</code> when specifying a subset of predictors. However, including some additional predictors results in the error. There doesn't seem to be anything particularly unique about the values of the predictor variables that cause the error when they are included.</p>
  
  <p>Any thoughts on why this may be occurring?</p>