diff --git a/sh/confirm b/sh/confirm
index d3d6513..7a4f61b 100755
--- a/sh/confirm
+++ b/sh/confirm
@@ -3,7 +3,7 @@
 # YES_BASH
 # NO_DASH
 
-if [ -n "${ZSH_VERSION:-}" ]; then
+if [ -n "$ZSH_VERSION" ]; then
     confirm() {
         [ $# -le 0 ] || { printf "%s\n" "$0: too many arguments" >&2; return 1; }
         read -q '?Continue? [y/N] '