diff --git a/lib/uri/ftp.rb b/lib/uri/ftp.rb
index 1c75e24..bb1b945 100644
--- a/lib/uri/ftp.rb
+++ b/lib/uri/ftp.rb
@@ -100,13 +100,8 @@ def self.build(args)
       # foo/bar       /foo/bar
       # /foo/bar      /%2Ffoo/bar
       #
-      if args.kind_of?(Array)
-        args[3] = '/' + args[3].sub(/^\//, '%2F')
-      else
-        args[:path] = '/' + args[:path].sub(/^\//, '%2F')
-      end
-
       tmp = Util::make_components_hash(self, args)
+      tmp[:path] = '/' + tmp[:path].sub(/^\//, '%2F')
 
       if tmp[:typecode]
         if tmp[:typecode].size == 1
diff --git a/test/uri/test_ftp.rb b/test/uri/test_ftp.rb
index 3ad7864..3a1d7a9 100644
--- a/test/uri/test_ftp.rb
+++ b/test/uri/test_ftp.rb
@@ -6,6 +6,19 @@ class URI::TestFTP < Test::Unit::TestCase
   def setup
   end
 
+
+  def test_build_does_not_mutate_components
+    components = {host: "example.test", path: "/folder"}
+    original = components.dup
+
+    first = URI::FTP.build(components)
+    second = URI::FTP.build(components)
+
+    assert_equal(original, components)
+    assert_equal(first, second)
+    assert_nothing_raised { URI::FTP.build(components.freeze) }
+  end
+
   def test_parse
     url = URI.parse('ftp://user:pass@host.com/abc/def')
     assert_kind_of(URI::FTP, url)