diff --git a/src/kotatsu.cr b/src/kotatsu.cr
index 49876c8..0ea1057 100644
--- a/src/kotatsu.cr
+++ b/src/kotatsu.cr
@@ -18,6 +18,12 @@ module Kotatsu
       puts VERSION
       exit
     end
+
+    parser.invalid_option do |flag|
+      STDERR.puts "ERROR! Not a valid option: #{flag}"
+      STDERR.puts "Use -h or --help for all available options"
+      exit(1)
+    end
   end
 
   webserv = HTTP::Server.new do |context|