Been bitten by this multiple times. I’ve been using Hanna as my default rdoc template following this article. This works fine with rdoc 2.3. Unfortunately rdoc seems to be undergoing some drastic changes at the moment and rdoc 2.4 breaks this lovely stuff.

So I probably have to do something like (all with sudo)

gem update --no-rdoc --no-ri
gem uninstall rdoc # choose 2.4
gem rdoc --all --no-ri

That last command can take ages to run so this is a real gem pain at the moment

Alternative

Go with the flow and use darkfish.

Change rdoc line in .gemrc to

rdoc: --line-numbers --format=darkfish

Then

gem update --no-rdoc --no-ri     
gem rdoc --all --no-ri