title: Effects package: Error using GLM binomial model
body: <p>I have been using the Effects package quite successfully in the past with lme4 but now I am stuck with a really simple example using glm. When fitting a binomial glm and then plotting the interaction using Effect I get an error message <code>"Error in Analyze.model(focal.predictors, mod, xlevels, default.levels,:the following predictor is not in the model: Freq*Gp"</code>. Below is a simple example that illustrates my problem.</p>
  
  <p>Thanks for your help.</p>
  
  <pre><code>Gp&lt;-c(rep("A",20),rep("B",20))
  Freq&lt;-c(rep(1,10),rep(2,10),rep(1,10),rep(2,10))
  Resp&lt;-sample(c(0,1),40, replace=T)
  data&lt;-data.frame(Gp)
  data$Freq&lt;-Freq
  data$Resp&lt;-Resp
  
  m2&lt;-glm(Resp~Freq*Gp, data,family=binomial)
  eff&lt;-Effect("Freq*Gp",m2)
  </code></pre>