diff --git a/lib/memoist.rb b/lib/memoist.rb
index ffbcab6..3d48b7e 100644
--- a/lib/memoist.rb
+++ b/lib/memoist.rb
@@ -96,7 +96,7 @@ def memoize(*method_names)
         include InstanceMethods
 
         if method_defined?(unmemoized_method)
-          raise "Already memoized #{method_name}"
+          raise AlreadyMemoizedError.new("Already memoized #{method_name}")
         end
         alias_method unmemoized_method, method_name
 
@@ -194,4 +194,6 @@ def #{method_name}(*args)
       end
     end
   end
+
+  class AlreadyMemoizedError < RuntimeError; end
 end
diff --git a/memoist.gemspec b/memoist.gemspec
index ce42aac..17d925a 100644
--- a/memoist.gemspec
+++ b/memoist.gemspec
@@ -9,7 +9,7 @@ Gem::Specification.new do |s|
   s.date = "2013-07-19"
   s.email = ["josh@joshpeek.com", "tarmo@itech.ee", "jeremy@bitsweat.net", "libc@mac.com", "fxn@hashref.com", "niels@herimedia.co", "wycats@gmail.com", "jeem@hughesorama.com", "john.pignata@gmail.com", "42@dmathieu.com", "jose.valim@gmail.com"]
   s.extra_rdoc_files = ["README.md"]
-  s.files = ["README.md", "test/benchmark/memoist_benchmark.rb", "test/memoist_test.rb", "test/test_helper.rb", "lib/memoist/core_ext/singleton_class.rb", "lib/memoist.rb"]
+  s.files = ["README.md", "test/memoist_test.rb", "test/test_helper.rb", "lib/memoist/core_ext/singleton_class.rb", "lib/memoist.rb"]
   s.homepage = "https://github.com/matthewrudy/memoist"
   s.licenses = ["MIT"]
   s.rdoc_options = ["--main", "README.md"]
diff --git a/test/memoist_test.rb b/test/memoist_test.rb
index ccad6a2..2e9af04 100644
--- a/test/memoist_test.rb
+++ b/test/memoist_test.rb
@@ -289,15 +289,15 @@ def test_object_memoized_module_methods
   end
 
   def test_double_memoization
-    assert_raise(RuntimeError) { Person.memoize :name }
+    assert_raise(Memoist::AlreadyMemoizedError) { Person.memoize :name }
     person = Person.new
     person.extend Memoist
-    assert_raise(RuntimeError) { person.memoize :name }
+    assert_raise(Memoist::AlreadyMemoizedError) { person.memoize :name }
 
     company = Company.new
     company.extend Memoist
     company.memoize :name
-    assert_raise(RuntimeError) { company.memoize :name }
+    assert_raise(Memoist::AlreadyMemoizedError) { company.memoize :name }
   end
 
   def test_double_memoization_with_identifier