Installing Jekyll SEO Tag causes Liquid Exception: Can't convert Hash into String

I am trying to install Jekyll SEO Tag as described here.

When I add {% seo %} to my head section, I am getting the following error - Liquid Exception: Can’t convert Hash into String. in /_layouts/default.html

head.html is located in the _includes folder and is included into the default.html as {% include head.html %}

What can cause this error?

Please post the full error message from running one of the following:

bundle exec jekyll build --trace

jekyll build --trace