diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index 5cc114781..a849d47e8 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -2691,7 +2691,7 @@ def responses(type = nil)
           warn(RESPONSES_DEPRECATION_MSG, uplevel: 1)
         when :frozen_dup
           synchronize {
-            responses = @responses.transform_values(&:freeze)
+            responses = @responses.transform_values { _1.dup.freeze }
             responses.default_proc = nil
             responses.default = [].freeze
             return responses.freeze
diff --git a/test/net/imap/test_imap_responses.rb b/test/net/imap/test_imap_responses.rb
index aaa61d436..3d1287635 100644
--- a/test/net/imap/test_imap_responses.rb
+++ b/test/net/imap/test_imap_responses.rb
@@ -166,13 +166,17 @@ def assert_responses_warn
         assert_equal [], imap.responses["FAKE"]
       end
       assert_empty stderr
-      # opt-in to future behavior
+      # default behavior since 0.6.0
       imap.config.responses_without_block = :frozen_dup
       stderr = EnvUtil.verbose_warning do
         assert imap.responses.frozen?
         assert imap.responses["CAPABILITY"].frozen?
         assert_equal(%w[IMAP4REV1 NAMESPACE MOVE IDLE UTF8=ACCEPT],
                      imap.responses["CAPABILITY"].last)
+        imap.responses do |r|
+          refute r.frozen?
+          refute r.values.any?(&:frozen?)
+        end
       end
       assert_empty stderr
     end