title: What does this error mean: HPE_INVALID_CONSTANT?
body: <p>I was receiving <code>HPE_INVALID_CONSTANT</code> when making HTTP requests with NodeJS the other day. They mysteriously stopped occurring recently, but I'm still baffled and curious what exactly <code>HPE_INVALID_CONSTANT</code> is trying to tell me.</p>
  
  <p>My google search results are a bit slanted towards NodeJS, so I'm not picking up anything that describes more generally what this means. It looks like a system level error since it's all caps.</p>