diff --git a/dnsapi/dns_knot.sh b/dnsapi/dns_knot.sh
index b6d1e0b6..094a6981 100644
--- a/dnsapi/dns_knot.sh
+++ b/dnsapi/dns_knot.sh
@@ -73,10 +73,10 @@ EOF
 _get_root() {
   domain=$1
   i="$(echo "$fulldomain" | tr '.' ' ' | wc -w)"
-  i=$(_math $i - 1)
+  i=$(_math "$i" - 1)
 
   while true; do
-    h=$(printf "%s" "$domain" | cut -d . -f $i-100)
+    h=$(printf "%s" "$domain" | cut -d . -f "$i"-100)
     if [ -z "$h" ]; then
       return 1
     fi