diff --git a/websocket_server/websocket_server.py b/websocket_server/websocket_server.py
index 9d1af5c..7eca370 100644
--- a/websocket_server/websocket_server.py
+++ b/websocket_server/websocket_server.py
@@ -8,6 +8,7 @@
 import logging
 from socket import error as SocketError
 import errno
+from threading import Thread
 
 if sys.version_info[0] < 3:
     from SocketServer import ThreadingMixIn, TCPServer, StreamRequestHandler
@@ -50,7 +51,12 @@
 
 # -------------------------------- API ---------------------------------
 
-class API():
+class API(Thread):
+    def __init__(self):
+        Thread.__init__(self)
+
+    def run(self):
+        self.run_forever()
 
     def run_forever(self):
         try:
@@ -61,7 +67,6 @@ def run_forever(self):
             logger.info("Server terminated.")
         except Exception as e:
             logger.error(str(e), exc_info=True)
-            exit(1)
 
     def new_client(self, client, server):
         pass
@@ -119,6 +124,7 @@ class WebsocketServer(ThreadingMixIn, TCPServer, API):
     id_counter = 0
 
     def __init__(self, port, host='127.0.0.1', loglevel=logging.WARNING):
+        API.__init__(self)
         logger.setLevel(loglevel)
         TCPServer.__init__(self, (host, port), WebSocketHandler)
         self.port = self.socket.getsockname()[1]
@@ -313,6 +319,11 @@ def read_http_headers(self):
     def handshake(self):
         headers = self.read_http_headers()
 
+        if 'upgrade' not in headers.keys():
+            self.request.send(self.make_upgrade_response().encode())
+            self.keep_alive = False
+            return
+
         try:
             assert headers['upgrade'].lower() == 'websocket'
         except AssertionError:
@@ -331,6 +342,17 @@ def handshake(self):
         self.valid_client = True
         self.server._new_client_(self)
 
+    @classmethod
+    def make_upgrade_response(cls):
+        return \
+            'HTTP/1.1 426 Upgrade Required\r\n'\
+            'Upgrade: HTTP/3.0\r\n'\
+            'Connection: Close\r\n'\
+            'Content-Encoding: text/plain\r\n'\
+            '\r\n'\
+            'This service requires use of the HTTP/3.0 protocol'
+
+
     @classmethod
     def make_handshake_response(cls, key):
         return \