diff --git a/lib/nmap/command.rb b/lib/nmap/command.rb
index c5170c0..332f4d3 100644
--- a/lib/nmap/command.rb
+++ b/lib/nmap/command.rb
@@ -734,7 +734,7 @@ def format(value)
 
       # FIREWALL/IDS EVASION AND SPOOFING:
       option '-f', name: :packet_fragments
-      option '--mtu'
+      option '--mtu', value: {type: Num.new, required: false}
       option '-D', name: :decoys, value: {type: List.new}
       option '-S', name: :spoof, value: true
       option '-e', name: :interface, value: true