diff --git a/lib/net/smtp.rb b/lib/net/smtp.rb
index cca06e6..ecaf480 100644
--- a/lib/net/smtp.rb
+++ b/lib/net/smtp.rb
@@ -626,7 +626,7 @@ def start(*args, helo: nil, user: nil, secret: nil, password: nil, authtype: nil
       secret ||= password || args[2]
       authtype ||= args[3]
       if defined?(OpenSSL::VERSION)
-        ssl_context_params = @ssl_context_params || {}
+        ssl_context_params = @ssl_context_params&.dup || {}
         unless ssl_context_params.has_key?(:verify_mode)
           ssl_context_params[:verify_mode] = @tls_verify ? OpenSSL::SSL::VERIFY_PEER : OpenSSL::SSL::VERIFY_NONE
         end
diff --git a/test/net/smtp/test_sslcontext.rb b/test/net/smtp/test_sslcontext.rb
index 7563d60..fa279da 100644
--- a/test/net/smtp/test_sslcontext.rb
+++ b/test/net/smtp/test_sslcontext.rb
@@ -222,5 +222,16 @@ def test_ssl_context_params_after_initialize
       smtp.start
       assert_equal(123, smtp.__ssl_context.timeout)
     end
+
+    def test_frozen_ssl_context_params
+      params = {}.freeze
+      smtp = SMTP.new("example.invalid", starttls: false)
+      smtp.ssl_context_params = params
+      smtp.define_singleton_method(:do_start) { |*| @started = true }
+      smtp.define_singleton_method(:do_finish) { @started = false }
+
+      assert_equal :started, smtp.start { :started }
+      assert_equal({}, params)
+    end
   end
 end