diff --git a/lib/fluent/config/types.rb b/lib/fluent/config/types.rb
index 4f5df084a4..f41d451b6a 100644
--- a/lib/fluent/config/types.rb
+++ b/lib/fluent/config/types.rb
@@ -64,7 +64,11 @@ def self.bool_value(str)
       end
     end
 
-    STRING_TYPE = Proc.new { |val, opts| val.to_s.force_encoding(Encoding::UTF_8) }
+    STRING_TYPE = Proc.new { |val, opts|
+      v = val.to_s
+      v = v.frozen? ? v.dup : v # config_param can't assume incoming string is mutable
+      v.force_encoding(Encoding::UTF_8)
+    }
     ENUM_TYPE = Proc.new { |val, opts|
       s = val.to_sym
       list = opts[:list]
diff --git a/test/config/test_configurable.rb b/test/config/test_configurable.rb
index 66d34b0847..2550065497 100644
--- a/test/config/test_configurable.rb
+++ b/test/config/test_configurable.rb
@@ -508,6 +508,11 @@ class TestConfigurable < ::Test::Unit::TestCase
           assert_instance_of(ConfigurableSpec::Base2, b2.configure(config_element("", "", {"name1" => "t1", "name5" => "t5", "opt3" => "a"})))
         end
 
+        test 'can accept frozen string' do
+          b2 = ConfigurableSpec::Base2.new
+          assert_instance_of(ConfigurableSpec::Base2, b2.configure(config_element("", "", {"name1" => "t1".freeze, "name5" => "t5", "opt3" => "a"})))
+        end
+
         test 'raise errors without any specifications for param without defaults' do
           b2 = ConfigurableSpec::Base2.new
           assert_raise(Fluent::ConfigError) { b2.configure(config_element("", "", {})) }