diff --git a/bootstrap b/bootstrap
index 0f97478..15e0846 100755
--- a/bootstrap
+++ b/bootstrap
@@ -5,7 +5,7 @@
 
 bsd_environment() {
 	# Based on https://github.com/rvm/rvm/blob/59fe3b39f0fb5ae01ed5b9aa187201080815ac16/scripts/functions/build_config_system#L123
-	if [[  -z "${AUTOCONF_VERSION:-}" ]]
+	if [ -z "${AUTOCONF_VERSION}" ]
 	then
 		export AUTOCONF_VERSION
 		AUTOCONF_VERSION="$(
@@ -17,7 +17,7 @@ bsd_environment() {
 		echo "Using autoconf version: $AUTOCONF_VERSION"
 	fi
 
-	if [[  -z "${AUTOMAKE_VERSION:-}" ]]
+	if [ -z "${AUTOMAKE_VERSION}" ]
 	then
 		export AUTOMAKE_VERSION
 		# FreeBSD might have automake-wrapper