diff --git a/lib/elixir/lib/uri.ex b/lib/elixir/lib/uri.ex
index 0a1858cf964..b6eeef526ad 100644
--- a/lib/elixir/lib/uri.ex
+++ b/lib/elixir/lib/uri.ex
@@ -656,16 +656,16 @@ defmodule URI do
         scheme = String.downcase(scheme, :ascii)
 
         case map do
-          %{port: port} when port != :undefined ->
+          %{port: port} when is_integer(port) ->
             %{uri | scheme: scheme}
 
           %{} ->
-            case default_port(scheme) do
-              nil -> %{uri | scheme: scheme}
-              port -> %{uri | scheme: scheme, port: port}
-            end
+            %{uri | scheme: scheme, port: default_port(scheme)}
         end
 
+      %{port: :undefined} ->
+        %{uri | port: nil}
+
       %{} ->
         uri
     end
diff --git a/lib/elixir/test/elixir/uri_test.exs b/lib/elixir/test/elixir/uri_test.exs
index e465cb323e3..8772ec97d3c 100644
--- a/lib/elixir/test/elixir/uri_test.exs
+++ b/lib/elixir/test/elixir/uri_test.exs
@@ -277,6 +277,32 @@ defmodule URITest do
     test "preserves an empty query" do
       assert URI.new!("http://foo.com/?").query == ""
     end
+
+    test "without scheme, undefined port after host translates to nil" do
+      assert URI.new!("//https://www.example.com") ==
+               %URI{
+                 scheme: nil,
+                 userinfo: nil,
+                 host: "https",
+                 port: nil,
+                 path: "//www.example.com",
+                 query: nil,
+                 fragment: nil
+               }
+    end
+
+    test "with scheme, undefined port after host translates to nil" do
+      assert URI.new!("myscheme://myhost:/path/info") ==
+               %URI{
+                 scheme: "myscheme",
+                 userinfo: nil,
+                 host: "myhost",
+                 port: nil,
+                 path: "/path/info",
+                 query: nil,
+                 fragment: nil
+               }
+    end
   end
 
   test "http://http://http://@http://http://?http://#http://" do