diff --git a/scripts/asn-query.nse b/scripts/asn-query.nse
index db6fd6d8c..fb4392f16 100644
--- a/scripts/asn-query.nse
+++ b/scripts/asn-query.nse
@@ -370,7 +370,7 @@ function get_prefix_length( range )
   last = ipOps.ip_to_bin(last)
 
   for pos = 1, #first do
-    if first:sub(pos, pos) ~= last:sub(pos, pos) then
+    if first:byte(pos) ~= last:byte(pos) then
       return pos - 1
     end
   end
diff --git a/scripts/whois-ip.nse b/scripts/whois-ip.nse
index edf12bc83..8e236e371 100644
--- a/scripts/whois-ip.nse
+++ b/scripts/whois-ip.nse
@@ -438,7 +438,7 @@ function get_prefix_length( range )
   last = ipOps.ip_to_bin(last)
 
   for pos = 1, #first do
-    if first:sub(pos, pos) ~= last:sub(pos, pos) then
+    if first:byte(pos) ~= last:byte(pos) then
       return pos - 1
     end
   end