diff --git a/spec/monkey_patches/alias_should_to_must.rb b/spec/monkey_patches/alias_should_to_must.rb
index <HASH>..<HASH> 100644
--- a/spec/monkey_patches/alias_should_to_must.rb
+++ b/spec/monkey_patches/alias_should_to_must.rb
@@ -2,4 +2,5 @@ class Object
     # This is necessary because the RAL has a 'should'
     # method.
     alias :must :should
+    alias :must_not :should_not
 end
Aliasing "must_not" just like we alias "must"