diff --git a/Running-Mastodon/Development-guide.md b/Running-Mastodon/Development-guide.md
index d6d8cfb7..b288ec32 100644
--- a/Running-Mastodon/Development-guide.md
+++ b/Running-Mastodon/Development-guide.md
@@ -110,6 +110,7 @@ These are self-contained instructions for setting up a development environment o
 ### Installation
 
 ```
+brew install libidn
 bundle install --with development
 yarn install --pure-lockfile
 gem install foreman --no-ri --no-rdoc