'Invalid theme folder' when trying to run Jekyll locally with mojombo-jekyll theme.

[tagervascio]

I've been running into a problem with the mojombo-jekyll theme when trying to run a fresh new Jekyll site locally. After running
jekyll new
and changing the theme in the Gemfile and _config.yml I run
bundle install
with no problems at all. However,
bundle exec jekyll serve
fails to serve the site and I get

Invalid theme folder: _sass
Invalid theme folder: _includes
jekyll 3.8.2 | Error:  method `/' not defined in Integer

I don't have any problems with the default theme, and these error messages aren't helping me out much, as those folders don't exist in the first place.
I'll keep working on this, but if anyone has any hunches I'd love to hear them.

[jangliss]

I can't answer the method '/' not defined in Integer message, but the Invalid theme folder is likely caused by the folder you have in your blog folder not existing in the theme folder. I couldn't find the "mojomobo-jekyll" theme, but I had the same error crop up on jekyll-clean-dark. I found that if I created an empty _sass folder in the gem folder, the error went away. The error is basically saying that the folder you have in your blog folder doesn't exist in the theme folder, so it doesn't know how to apply the theme code to it. It shouldn't cause a failure however.

[cetinajero]

This error is only present when using Jekyll from v3.8.0 to v3.8.5 as it was fixed in v3.8.6 and further via jekyll/jekyll#7679.

It got my attention again because the fix was also merged yesterday to GitHub Pages via github/pages-gem#649 (comment).