`zsh` uses [square brackets for globbing / pattern matching](http://zsh.sourceforge.net/Guide/zshguide05.html#l137). That means that if you need to pass literal square brackets as an argument to a command, you either need to escape them or quote the argument like this: ```python pip install 'requests[security]' ``` If you want to disable [globbing](http://en.wikipedia.org/wiki/Glob_%28programming%29) for the `pip` command permanently, you can do so by adding this to your `~/.zshrc`: ```python alias pip='noglob pip' ``` If you don't use quoted arguments, you would get an error like this: ```bash zsh: no matches found: requests[security] ```