title: While testing package I get: invalid subscript type 'list'
body: <p>While testing my package, I get an error: </p>
  
  <pre><code>invalid subscript type 'list'
  </code></pre>
  
  <p>I cannot reproduce the error during interactive testing, only while running tests on my package (ie, Ctrl + Shift + T or Ctrl + Alt + F7 in RStudio/Server). </p>
  
  <p>R associates the error with a typical data.table style join. </p>
  
  <pre><code>DT &lt;- DT[ODT]
  # where DT is a data.table and ODT is another data.table.
  </code></pre>