2 files changed, 6 insertions, 1 deletions
diff --git a/lib/random/formatter.rb b/lib/random/formatter.rb
index 2b5cf718ad..4ecd6ad027 100644
--- a/lib/random/formatter.rb
+++ b/lib/random/formatter.rb
@@ -340,7 +340,7 @@ module Random::Formatter
   end
 
   # The default character list for #alphanumeric.
-  ALPHANUMERIC = [*'A'..'Z', *'a'..'z', *'0'..'9']
+  ALPHANUMERIC = [*'A'..'Z', *'a'..'z', *'0'..'9'].map(&:freeze).freeze
 
   # Generate a random alphanumeric string.
   #
diff --git a/test/ruby/test_random_formatter.rb b/test/ruby/test_random_formatter.rb
index f927522d96..784fbfc2b8 100644
--- a/test/ruby/test_random_formatter.rb
+++ b/test/ruby/test_random_formatter.rb
@@ -165,6 +165,11 @@ module Random::Formatter
     def setup
       @it = Random
     end
+
+    def test_alphanumeric_frozen
+      assert_predicate @it::Formatter::ALPHANUMERIC, :frozen?
+      assert @it::Formatter::ALPHANUMERIC.all?(&:frozen?)
+    end
   end
 
   class TestInstanceMethods < Test::Unit::TestCase