title: Argument needs to be of the class StMoMoData
body: <p>Hi I'm going by the steps in <a href="https://cran.r-project.org/web/packages/StMoMo/vignettes/StMoMoVignette.pdf" rel="nofollow noreferrer">this file</a> to fit a CBD Model to US Male data, sourced from the HMD. Currently I have the error that my argument data is not of the class StMoMoData. </p>
  
  <pre><code>CBDfit &lt;- fit(CBD, data = USData, ages.fit = 60:89)
  Error in fit.StMoMo(CBD, data = USData, ages.fit = 60:89) : 
    Argument data needs to be of class StMoMoData.
  </code></pre>
  
  <p>I have tried downloading the HMD data straight from R using the demography package. I have also tried saving the HMD data as a .csv file and reading that into R. Any other thoughts?</p>