R help - Apr 2009 - error message in plm package

Dear R help, 
      I use the package plm  the function plm() to analyse a panel data and 
   estimate a fixeffect model. 
   I use the code as follow : 
    fe <- plm(y~x+z, data, model = "within")
  But I had a error message 
  Error in model.frame.default(form, ...) : object is not a matrix
  I don't what's wrong. Beacuse others use the same codes and data,the
code
can work.
   I consult the paper plm.pdf , but I can't find any answer. 
  

   Thanks, 
   I really would appreciate some suggestions.
   Helen Chen 
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