This question (and it's answers) on stackoverflow seems it may match what you are seeing: Installing Rails on Mavericks (http://stackoverflow.com/questions/19580685/installing-rails-on-mavericks) I'm basing this solely on the error you are seeing, though since I don't know if you have updated (or have) xcode, or have updated to Mavericks.

Welcome to the Treehouse Community

Want to collaborate on code errors? Have bugs you need feedback on? Looking for an extra set of eyes on your latest project? Get support with fellow developers, designers, and programmers of all backgrounds and skill levels here with the Treehouse Community! While you're at it, check out some resources Treehouse students have shared here.

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and join thousands of Treehouse students and alumni in the community today.

Start your free trial