title: What does "Error in namespaceExport(ns, exports) : undefined exports" mean?
body: <p>When building a package, I got the error</p>
  
  <pre><code>Error in namespaceExport(ns, exports) : 
    undefined exports: FooBarBaz
  </code></pre>
  
  <p>What does this mean, and how do I fix it?</p>