diff --git a/acme.sh b/acme.sh
index 79afb23..705f5a9 100755
--- a/acme.sh
+++ b/acme.sh
@@ -259,7 +259,7 @@ _exists() {
 
 #a + b
 _math() {
-  $(( "$@" ))
+  printf "%s" "$(( $@ ))"
 }
 
 _h_char_2_dec() {