diff --git a/lib/uh/wm.rb b/lib/uh/wm.rb
index 16b9951..6199790 100644
--- a/lib/uh/wm.rb
+++ b/lib/uh/wm.rb
@@ -1,3 +1,11 @@
 require 'uh'
 
 require 'uh/wm/cli'
+
+module Uh
+  module WM
+    Error         = Class.new(StandardError)
+    RuntimeError  = Class.new(RuntimeError)
+    ArgumentError = Class.new(Error)
+  end
+end