title: Error in model.frame.default(object, data, xlev = xlev) : object is not a matrix
body: <p>3 days old to R and can't figure out what I'm doing wrong.  I'm trying to send some columns with two way interactions into a glmnet cox model.  I have some data.frame() called dtable</p>
  
  <p><em>Edit to make the code reproducible</em></p>
  
  <pre><code>xs&lt;-c("Col1", "Col2", "Col3")
  v&lt;-c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, NA, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, NA, 25, 26, 27, 28, 29, 30)
  df&lt;-data.frame(matrix(v,ncol=3))
  dm&lt;-as.matrix(df)
  dm&lt;-matrix(dm[complete.cases(dm)], ncol=3)
  colnames(dm)&lt;-xs
  dfdata&lt;-data.frame(dm)
  f&lt;-as.formula(time~.*.)
  xmatrix&lt;-model.matrix(f, dfdata)[,-1]
  </code></pre>
  
  <p>When I run this I get the error</p>
  
  <pre><code>Error in model.frame.default(object, data, xlev = xlev) :    
  object is not a matrix
  </code></pre>
  
  <p>Thanks in advance</p>