title: Cannot introduce offset into negative binomial regression
body: <p>I'm performing a count data analysis in R, dealing with a data called 'doctor' which is:</p>
  <pre><code>   V2  V3        L4 V5
  1   1  32 10.866795  1
  2   2 104 10.674706  1
  3   3 206 10.261581  1
  4   4 186  9.446440  1
  5   5 102  8.578665  1
  6   1   2  9.841080  2
  7   2  12  9.275472  2
  8   3  28  8.649974  2
  9   4  28  7.857481  2
  10  5  31  7.287561  2
  </code></pre>
  <p>The best model was V3~V2+L4+V5+V2:L4:V5 using stepwise AIC. Now I want to set L4 as the offset and perform negative binomial regression including the interaction, so I used the code <code>nbinom=glm.nb(V3~V2+V5+V2:V5,offset=L4)</code> but get this error message that says <code>Error in glm.control(...) : unused argument (offset = L4)</code>. What have I done wrong here?</p>