diff --git a/lib/protocol/rack/adapter/rack2.rb b/lib/protocol/rack/adapter/rack2.rb
index ca98c63..01eee07 100644
--- a/lib/protocol/rack/adapter/rack2.rb
+++ b/lib/protocol/rack/adapter/rack2.rb
@@ -59,8 +59,12 @@ def make_environment(request)
 						
 						# I'm not sure what sane defaults should be here:
 						CGI::SERVER_NAME => server_name,
-						CGI::SERVER_PORT => server_port,
 					}
+
+					# SERVER_PORT is optional but must not be set if it is not present.
+					if server_port
+						env[CGI::SERVER_PORT] = server_port
+					end
 					
 					self.unwrap_request(request, env)
 					
diff --git a/lib/protocol/rack/adapter/rack3.rb b/lib/protocol/rack/adapter/rack3.rb
index eb00e5f..b43febb 100644
--- a/lib/protocol/rack/adapter/rack3.rb
+++ b/lib/protocol/rack/adapter/rack3.rb
@@ -55,8 +55,12 @@ def make_environment(request)
 						
 						# I'm not sure what sane defaults should be here:
 						CGI::SERVER_NAME => server_name,
-						CGI::SERVER_PORT => server_port,
 					}
+
+					# SERVER_PORT is optional but must not be set if it is not present.
+					if server_port
+						env[CGI::SERVER_PORT] = server_port
+					end
 					
 					self.unwrap_request(request, env)
 					
diff --git a/lib/protocol/rack/adapter/rack31.rb b/lib/protocol/rack/adapter/rack31.rb
index 0e1aae5..b2cf73e 100644
--- a/lib/protocol/rack/adapter/rack31.rb
+++ b/lib/protocol/rack/adapter/rack31.rb
@@ -46,8 +46,12 @@ def make_environment(request)
 						
 						# I'm not sure what sane defaults should be here:
 						CGI::SERVER_NAME => server_name,
-						CGI::SERVER_PORT => server_port,
 					}
+
+					# SERVER_PORT is optional but must not be set if it is not present.
+					if server_port
+						env[CGI::SERVER_PORT] = server_port
+					end
 					
 					if body = request.body
 						if body.empty?
diff --git a/test/protocol/rack/adapter/generic.rb b/test/protocol/rack/adapter/generic.rb
index f6f83bb..125780f 100644
--- a/test/protocol/rack/adapter/generic.rb
+++ b/test/protocol/rack/adapter/generic.rb
@@ -35,6 +35,18 @@
 			expect(env).to have_keys("CONTENT_TYPE" => be == "text/plain")
 		end
 	end
+
+	with "app server without SERVER_PORT" do
+    let(:request) do
+      Protocol::HTTP::Request.new("https", "example.com", "GET", "/", "http/1.1", Protocol::HTTP::Headers[{"accept" => "text/html"}], nil)
+    end
+
+    it "does not include SERVER_PORT in the Rack environment" do
+      env = adapter.make_environment(request)
+      
+      expect(env).not.to have_keys(Protocol::Rack::CGI::SERVER_PORT)
+    end
+  end
 	
 	with "a app that returns nil" do
 		include DisableConsoleContext