diff --git a/lib/uri/ftp.rb b/lib/uri/ftp.rb
index 1c75e24..0270af5 100644
--- a/lib/uri/ftp.rb
+++ b/lib/uri/ftp.rb
@@ -249,14 +249,11 @@ def set_path(v)
 
     # Returns a String representation of the URI::FTP.
     def to_s
-      save_path = nil
-      if @typecode
-        save_path = @path
-        @path = @path + TYPECODE_PREFIX + @typecode
-      end
       str = super
       if @typecode
-        @path = save_path
+        suffix_length = (@query ? @query.length + 1 : 0) +
+          (@fragment ? @fragment.length + 1 : 0)
+        str.insert(-suffix_length - 1, TYPECODE_PREFIX + @typecode)
       end
 
       return str
diff --git a/test/uri/test_ftp.rb b/test/uri/test_ftp.rb
index 3ad7864..d35f209 100644
--- a/test/uri/test_ftp.rb
+++ b/test/uri/test_ftp.rb
@@ -6,6 +6,15 @@ class URI::TestFTP < Test::Unit::TestCase
   def setup
   end
 
+
+  def test_to_s_does_not_mutate_frozen_ftp_uri
+    uri = URI.parse("ftp://example.test/file;type=i?download=1#section").freeze
+
+    2.times do
+      assert_equal("ftp://example.test/file;type=i?download=1#section", uri.to_s)
+    end
+  end
+
   def test_parse
     url = URI.parse('ftp://user:pass@host.com/abc/def')
     assert_kind_of(URI::FTP, url)