diff --git a/scripts/prompt_direnv_macos.sh b/scripts/prompt_direnv_macos.sh
--- a/scripts/prompt_direnv_macos.sh
+++ b/scripts/prompt_direnv_macos.sh
@@ -21,7 +21,7 @@
 # `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