I went to update a Middleman project's gems today.
After I did, I noticed
middleman server exited with the following error:
Assertion failed: (!STR_EMBED_P(shared)), function str_new_frozen, file string.c, line 1075.
This is an internal Ruby error generated from within its C source. I found a discussion about the topic in the Ruby bug tracker, but that wasn't much help.
Once I modified my Gemfile to use Ruby 2.3.1 and re-bundled my gems,
server worked just fine.
If you're running into this problem, try upgrading to a more recent Ruby and running Middleman on it. There's a chance this bug has nothing to do with Middleman and could be due to another gem, but I figured I'd mention it here in case someone else is having the same problem.