diff --git a/nselib/stdnse.lua b/nselib/stdnse.lua
index ee56f134d..7ad5a5c22 100644
--- a/nselib/stdnse.lua
+++ b/nselib/stdnse.lua
@@ -158,6 +158,7 @@ end
 --@return hexadecimal encoded string
 function tohex( s, options ) 
   options = options or {}
+  local separator = options.separator
   local hex
 
   if type( s ) == 'number' then
@@ -168,16 +169,19 @@ function tohex( s, options )
     error( "Type not supported in tohex(): " .. type(s), 2 )
   end
 
-  if options.separator then
+  -- format hex if we got a separator
+  if separator then
     local group = options.group or 2
     local fmt_table = {}
+    -- split hex in group-size chunks
     for i=#hex,1,-group do
       -- table index must be consecutive otherwise table.concat won't work
       fmt_table[ceil(i/group)] = hex:sub(max(i-group+1,1),i)
     end
 
-    hex = concat( fmt_table, options.separator )
+    hex = concat( fmt_table, separator )
   end
+
   return hex
 end