diff --git a/terraform/nix-build/nix-build.sh b/terraform/nix-build/nix-build.sh
index 01c7927..d8d3210 100755
--- a/terraform/nix-build/nix-build.sh
+++ b/terraform/nix-build/nix-build.sh
@@ -3,7 +3,7 @@ set -efu
 
 declare file attribute
 eval "$(jq -r '@sh "attribute=\(.attribute) file=\(.file)"')"
-if [[ -e ${file+x} ]]; then
+if [[ -n ${file-} ]] && [[ -e ${file-} ]]; then
   out=$(nix build --no-link --json -f "$file" "$attribute")
   printf '%s' "$out" | jq -c '.[].outputs'
 else