diff --git a/share/spack/setup-env.sh b/share/spack/setup-env.sh
index f602ea627ee..943db726120 100755
--- a/share/spack/setup-env.sh
+++ b/share/spack/setup-env.sh
@@ -58,7 +58,7 @@
 
 function spack {
     # Zsh does not do word splitting by default, this enables it for this function only
-    if [ -n "$ZSH_VERSION" ]; then
+    if [ -n "${ZSH_VERSION:-}" ]; then
         emulate -L sh
     fi