diff --git a/install.sh b/install.sh
index 8d5518b..7bf0745 100755
--- a/install.sh
+++ b/install.sh
@@ -31,7 +31,7 @@ function bootstrap {
 ## MAIN
 
 [[ "$UID" -ne 0 ]] && echo 'root privilege required' && exit 1
-if [[ -v MIAOU_BASH_DIR ]]; then
+if [[ -v MIAOU_BASH_DIR && -d "$MIAOU_BASH_DIR" ]]; then
   miaou-bash "$MIAOU_BASH_DIR/lib/self-upgrade.bash"
 else
   bootstrap