diff --git a/lib/uri/generic.rb b/lib/uri/generic.rb
index 2a066a4cb6..044d408f50 100644
--- a/lib/uri/generic.rb
+++ b/lib/uri/generic.rb
@@ -1341,7 +1341,7 @@ module URI
       if @opaque
         str << @opaque
       else
-        if @host
+        if @host || %w[file postgres].include?(@scheme)
           str << '//'
         end
         if self.userinfo
diff --git a/test/uri/test_generic.rb b/test/uri/test_generic.rb
index aa782db27e..b77717f949 100644
--- a/test/uri/test_generic.rb
+++ b/test/uri/test_generic.rb
@@ -20,6 +20,10 @@ class URI::TestGeneric < Test::Unit::TestCase
     str = URI(exp).to_s
     assert_equal exp, str
     assert_not_predicate str, :frozen?, '[ruby-core:71785] [Bug #11759]'
+
+    assert_equal "file:///foo", URI("file:///foo").to_s
+    assert_equal "postgres:///foo", URI("postgres:///foo").to_s
+    assert_equal "http:/foo", URI("http:///foo").to_s
   end
 
   def test_parse