diff --git a/sorted_set.gemspec b/sorted_set.gemspec
index e66f598..ab38c44 100644
--- a/sorted_set.gemspec
+++ b/sorted_set.gemspec
@@ -8,7 +8,7 @@ Gem::Specification.new do |spec|
   spec.description   = %q{Implements a variant of Set whose elements are sorted in ascending order}
   spec.homepage      = "https://github.com/knu/sorted_set"
   spec.license       = "BSD-2-Clause"
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.4.0")
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage
diff --git a/test/test_sorted_set.rb b/test/test_sorted_set.rb
index 3b7622a..ad100c1 100644
--- a/test/test_sorted_set.rb
+++ b/test/test_sorted_set.rb
@@ -2,6 +2,8 @@
 require 'sorted_set'
 
 class TC_SortedSet < Test::Unit::TestCase
+  FrozenError = RuntimeError unless defined?(FrozenError)
+
   def test_sortedset
     s = SortedSet[4,5,3,1,2]