This is one of those posts that serve only to remind me of something in the future. My Emacs configuration was working well on one machine. Whenever I opened dired on another machine, though, this error was returned:
wrong type argument: stringp, nil
Finally, I remembered that I had required GNU ls in my Emacs settings, and had not installed it on the machine in question. A quick “brew install coreutils” to install the GNU Command Line Tools with Homebrew did the trick.