title: Error in <my code> : target of assignment expands to non-language object
body: <p>I received the error</p>
  
  <pre><code>Error in &lt;my code&gt; : target of assignment expands to non-language object
  </code></pre>
  
  <p>or</p>
  
  <pre><code>Error in &lt;my code&gt; : invalid (do_set) left-hand side to assignment
  </code></pre>
  
  <p>or</p>
  
  <pre><code>Error in &lt;my code&gt; : invalid (NULL) left side of assignment
  </code></pre>
  
  <p>What does it mean, and how do I prevent it?</p>