diff --git a/CHANGELOG.md b/CHANGELOG.md
index cb5a411..00ff697 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 * Fix error `singleton can't be dumped (TypeError)` at the end of dynamic analysis ([#17](https://github.com/yujinakayama/transpec/issues/17))
 * Do not copy pseudo files (device, socket, etc.) in dynamic analysis ([#17](https://github.com/yujinakayama/transpec/issues/17))
+* Fix error `undefined method receive_messages_available?` while conversion ([#17](https://github.com/yujinakayama/transpec/issues/17))
 
 ## v1.2.1
 
diff --git a/lib/transpec.rb b/lib/transpec.rb
index 15ef531..3b2950d 100644
--- a/lib/transpec.rb
+++ b/lib/transpec.rb
@@ -16,7 +16,7 @@ def self.required_rspec_version
       if rspec_dependency
         rspec_requirement = rspec_dependency.requirement
         gem_version = rspec_requirement.requirements.first.find { |r| r.is_a?(Gem::Version) }
-        RSpecVersion.new(gem_version.to_s)
+        RSpecVersion.new(gem_version)
       else
         # Using development version of RSpec with Bundler.
         current_rspec_version
diff --git a/lib/transpec/rspec_version.rb b/lib/transpec/rspec_version.rb
index 875694b..6166ba9 100644
--- a/lib/transpec/rspec_version.rb
+++ b/lib/transpec/rspec_version.rb
@@ -3,7 +3,11 @@
 require 'transpec'
 
 module Transpec
-  class RSpecVersion < Gem::Version
+  # Gem::Version caches instances with class variable @@all,
+  # so we should not inherit it.
+  class RSpecVersion
+    include Comparable
+
     # http://www.ruby-doc.org/stdlib-2.0.0/libdoc/rubygems/rdoc/Gem/Version.html
     #
     # If any part contains letters (currently only a-z are supported) then that version is
@@ -11,15 +15,33 @@ class RSpecVersion < Gem::Version
     # Prerelease parts are sorted alphabetically using the normal Ruby string sorting rules.
     # If a prerelease part contains both letters and numbers, it will be broken into multiple parts
     # to provide expected sort behavior (1.0.a10 becomes 1.0.a.10, and is greater than 1.0.a9).
-    VERSION_2_99 = new('2.99.aaaaaaaaaa')
-    VERSION_3_0  = new('3.0.aaaaaaaaaa')
+    GEM_VERSION_2_99 = Gem::Version.new('2.99.aaaaaaaaaa')
+    GEM_VERSION_3_0  = Gem::Version.new('3.0.aaaaaaaaaa')
+
+    attr_reader :gem_version
+
+    def initialize(version)
+      @gem_version = if version.is_a?(Gem::Version)
+                       version
+                     else
+                       Gem::Version.new(version)
+                     end
+    end
 
     def be_truthy_available?
-      self >= VERSION_2_99
+      @gem_version >= GEM_VERSION_2_99
     end
 
     def receive_messages_available?
-      self >= VERSION_3_0
+      @gem_version >= GEM_VERSION_3_0
+    end
+
+    def <=>(other)
+      @gem_version <=> other.gem_version
+    end
+
+    def to_s
+      @gem_version.to_s
     end
   end
 end