title: What is the meaning of 'attempt to index upvalue'
body: <p>I am taking my first steps programming in Lua and get this error when I run my script:</p>
  
  <pre><code>attempt to index upvalue 'base' (a function value)
  </code></pre>
  
  <p>It's probably due to something very basic that I haven't grasped yet, but I can't find any good information about it when googling. Could someone explain to me what it means?</p>