Puzzling error message

[alexander-yevsyukov]

I got the following error message:

jekyll 3.8.7 | Error:  no implicit conversion of nil into String
/Users/sanders/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/bundler/gems/embed-code-eb18b3b9c6c5/lib/commands/embedding.rb:256:in `+': no implicit conversion of nil into String (TypeError)
        from /Users/sanders/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/bundler/gems/embed-code-eb18b3b9c6c5/lib/commands/embedding.rb:256:in `block in render_sample'
        from /Users/sanders/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/bundler/gems/embed-code-eb18b3b9c6c5/lib/commands/embedding.rb:254:in `each'
        from /Users/sanders/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/bundler/gems/embed-code-eb18b3b9c6c5/lib/commands/embedding.rb:254:in `render_sample'

The problem is that it does not give any hint on where the error happened. What do I need to change in the markdown?
What command was wrong?