diff --git a/usr.sbin/jail/tests/jail_basic_test.sh b/usr.sbin/jail/tests/jail_basic_test.sh
index c781eed7875..cb50a94a45e 100755
--- a/usr.sbin/jail/tests/jail_basic_test.sh
+++ b/usr.sbin/jail/tests/jail_basic_test.sh
@@ -186,11 +186,12 @@ find_unused_jid()
 			atf_skip "System has too many jail, cannot find free slot"
 		fi
 
-		jid=$((jid + 1))
+		: $(( jid += 1 ))
 	done
 
 	echo "$jid" | tee -a jails.lst
 }
+
 clean_jails()
 {
 	if [ ! -s jails.lst ]; then