diff --git a/lib/pitchfork/http_parser.rb b/lib/pitchfork/http_parser.rb
index 705d181a..6f0fc4c7 100644
--- a/lib/pitchfork/http_parser.rb
+++ b/lib/pitchfork/http_parser.rb
@@ -93,6 +93,9 @@ def read(socket)
       e['pitchfork.socket'] = socket
       e['rack.hijack'] = self
 
+      # We don't support connection upgrade:
+      e.delete('HTTP_UPGRADE')
+
       e.merge!(DEFAULTS)
     end
 
diff --git a/test/integration/test_http_basic.rb b/test/integration/test_http_basic.rb
index d5a65f43..2cbafc4f 100644
--- a/test/integration/test_http_basic.rb
+++ b/test/integration/test_http_basic.rb
@@ -136,4 +136,28 @@ def test_write_on_close
 
     assert_clean_shutdown(pid)
   end
+
+  def test_http_upgrade
+    addr, port = unused_port
+
+    pid = spawn_server(app: File.join(ROOT, "test/integration/upgrade.ru"), config: <<~CONFIG)
+      listen "#{addr}:#{port}"
+      worker_processes 1
+    CONFIG
+
+    assert_healthy("http://#{addr}:#{port}")
+
+    Net::HTTP.start(addr, port) do |http|
+      request = Net::HTTP::Get.new("/")
+      request["Connection"] = "Upgrade"
+      request["Upgrade"] = "websocket"
+
+      # It should not be connection upgrade:
+      response = http.request(request)
+      assert_equal "200", response.code
+      assert_equal "Normal response", response.body
+    end
+
+    assert_clean_shutdown(pid)
+  end
 end
diff --git a/test/integration/upgrade.ru b/test/integration/upgrade.ru
new file mode 100644
index 00000000..edf305f5
--- /dev/null
+++ b/test/integration/upgrade.ru
@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+run lambda { |env|
+  if env['HTTP_UPGRADE']
+    [404, {}, ["Upgrade not supported"]]
+  else
+    [200, {}, ["Normal response"]]
+  end
+}