diff --git a/scripts/prompt_direnv_macos.sh b/scripts/prompt_direnv_macos.sh
index ec2a2ee1a..18bc56ced 100755
--- a/scripts/prompt_direnv_macos.sh
+++ b/scripts/prompt_direnv_macos.sh
@@ -1,79 +1,79 @@
 #!/usr/bin/env bash
 
 set -euo pipefail
 
 COMM_CACHE="${XDG_CACHE_HOME:-$HOME/Library/Caches}/app.comm"
 mkdir -p "$COMM_CACHE"
 COMM_DIRENV="$COMM_CACHE/install-direnv"
 
 if command -v direnv >/dev/null || [[ -s "$COMM_DIRENV" ]]; then
   # Already using direnv or anwsered no previously. Exit immediately.
   exit 0
 fi
 
 # Currently, this script only works on macOS as it assumes homebrew usage
 if ! [[ "$OSTYPE" == 'darwin'* ]]; then
   echo "This script is only meant to be ran on macOS" >&2
   exit 1
 fi
 
 # Check if in an interactive shell
 # `test -t` tests if a file descriptor is open, 0 being stdin
 # Normally, a non-interactive shell will not have 0 FD bound
 # However, Buildkite still has 0 FD bound, so check if PS1 is empty
-if [[ ! -t 0 ]] || [[ -z "$PS1" ]]; then
+if [[ ! -t 0 ]] || [[ -z "${PS1-}" ]]; then
   exit 0
 fi
 
 if [[ ! -e "${COMM_DIRENV}" ]]; then
   echo "Direnv is a tool which will automatically setup the development environment upon entering the comm/ directory."
   read -r \
     -p "Would you like to install direnv? [y/N]" \
     response
   case "$response" in
     [yY][eE][sS]|[yY])
       echo "1" > "${COMM_DIRENV}"
       ;;
     *)
       touch "${COMM_DIRENV}"
       exit 1
       ;;
   esac
 fi
 
 brew install direnv
 
 # A more recent version of bash is required for nix-direnv to work correctly
 # Default version on macOS is bash 3.2. Output taken from '/bin/bash --version'
 # shellcheck disable=SC2076
 if [[ "$(bash --version)" =~ "GNU bash, version 3.2" ]]; then
   brew install bash
 fi
 
 # Install the hook for a given shell in the related file.
 install_direnv_hook() {
   local shell="$1"
   local source_file="$2"
 
   # Skip hook installation of shells which don't exist
   if ! command -v "$shell" >/dev/null; then
     return 0
   fi
 
   # If the file already mentions direnv, then hook is likely already installed
   if [[ ! -e "$source_file" ]] \
       || ! grep "direnv" "$source_file" >/dev/null; then
     # shellcheck disable=SC2016
     echo "eval \"\$(direnv hook $shell)\"" >> "$source_file"
   fi
 
 }
 
 # ~/.zshenv would be preferred location to install the hook, however, it
 # gets sourced before ~/.zprofile for login shells; and since
 # Homebrew asks you to install its hook in ~/.zprofile, we need to
 # follow the same convention.
 install_direnv_hook zsh ~/.zprofile
 install_direnv_hook zsh ~/.zshrc
 install_direnv_hook bash ~/.bash_profile
 install_direnv_hook bash ~/.bashrc