Jekyll Complains "GitHub Metadata: No GitHub API authentication could be found."

When building a Jekyll site, I encountered the following error message,

$ bundle exec jekyll build
Configuration file: _config.yml
            Source: foosite/
       Destination: _site
 Incremental build: disabled. Enable with --incremental
      Generating...
   GitHub Metadata: No GitHub API authentication could be found. 
   Some fields may be missing or have incorrect data.

The sources of this error are many. What caused the error in my case? It is actually the combination of the two facts:

1. I used the Jekyll SEO Tags plugin, and
2. I did not include a description value in the _config.yml file. 

After my adding a description map to the _config.yml file, the problem went away, e.g.,

$ cat _config.yml
...
description: this is my awesome site. 
...