diff --git a/lib/mutant/mutator.rb b/lib/mutant/mutator.rb
index 2c527519..f0ba147a 100644
--- a/lib/mutant/mutator.rb
+++ b/lib/mutant/mutator.rb
@@ -5,16 +5,16 @@ module Mutant
 
     # Run mutator on input
     #
-    # @param [Object] input
+    # @param [Parser::AST::Node] node
     # @param [#call] block
     #
     # @return [self]
     #
     # @api private
     #
-    def self.each(input, &block)
-      return to_enum(__method__, input) unless block_given?
-      Registry.lookup(input.class).new(input, block)
+    def self.each(node, &block)
+      return to_enum(__method__, node) unless block_given?
+      Registry.lookup(node).new(node, block)
 
       self
     end
diff --git a/lib/mutant/mutator/registry.rb b/lib/mutant/mutator/registry.rb
index 87411f68..ec9a357b 100644
--- a/lib/mutant/mutator/registry.rb
+++ b/lib/mutant/mutator/registry.rb
@@ -27,9 +27,9 @@ module Mutant
         self
       end
 
-      # Lookup mutator class for AST node class
+      # Lookup mutator class for node 
       #
-      # @param [Class] ast_class
+      # @param [Parser::AST::Node] node
       #
       # @return [Class]
       #
@@ -38,9 +38,10 @@ module Mutant
       #
       # @api private
       #
-      def self.lookup(ast_class)
-        registry.fetch(ast_class) do
-          raise ArgumentError,"No mutator to handle: #{ast_class.inspect}"
+      def self.lookup(node)
+        type = node.type
+        registry.fetch(type) do
+          raise ArgumentError,"No mutator to handle: #{type.inspect}"
         end
       end
     end
diff --git a/spec/shared/mutator_behavior.rb b/spec/shared/mutator_behavior.rb
index 5c58ae54..1aac3d8b 100644
--- a/spec/shared/mutator_behavior.rb
+++ b/spec/shared/mutator_behavior.rb
@@ -1,11 +1,11 @@
 shared_examples_for 'a mutator' do
   subject { object.each(node) { |item| yields << item } }
 
-  let(:yields)        { []              }
-  let(:object)        { described_class }
+  let(:yields) { []              }
+  let(:object) { described_class }
 
   unless instance_methods.map(&:to_s).include?('node')
-    let(:node)          { parse(source) }
+    let(:node) { parse(source) }
   end
 
   it_should_behave_like 'a command method'