diff --git a/build.sh b/build.sh
index 286f914..9f78ecb 100755
--- a/build.sh
+++ b/build.sh
@@ -2,7 +2,7 @@
 
 image="${1}"
 
-if [[ -z "${image}" ]]; then
+if [ -z "${image}" ]; then
     echo "usage: ${0} image" >&2
     exit 1
 fi