diff --git a/lib/json_logic.rb b/lib/json_logic.rb
index 3bb256b..e80b329 100644
--- a/lib/json_logic.rb
+++ b/lib/json_logic.rb
@@ -61,7 +61,7 @@ def self.filter(logic, data)
   end
 
   def self.add_operation(operator, function)
-    Operation.class.send(:define_method, operator) do |v, d|
+    Operation.define_singleton_method(operator) do |v, d|
       function.call(v, d)
     end
   end
diff --git a/lib/json_logic/operation.rb b/lib/json_logic/operation.rb
index 5cec5b6..b909672 100644
--- a/lib/json_logic/operation.rb
+++ b/lib/json_logic/operation.rb
@@ -144,11 +144,5 @@ def self.is_standard?(operator)
     def self.is_iterable?(operator)
       ['filter', 'some', 'all', 'none', 'in', 'map', 'reduce'].include?(operator.to_s)
     end
-
-    def self.add_operation(operator, function)
-      self.class.send(:define_method, operator) do |v, d|
-        function.call(v, d)
-      end
-    end
   end
 end