diff --git a/tests/protocols/test_http.py b/tests/protocols/test_http.py
index c1cd9dc4c..08a51be3a 100644
--- a/tests/protocols/test_http.py
+++ b/tests/protocols/test_http.py
@@ -57,6 +57,12 @@
     b""
 ])
 
+INVALID_REQUEST = b"\r\n".join([
+    b"GET /?x=y z HTTP/1.1",  # bad space character
+    b"Host: example.org",
+    b"",
+    b""
+])
 
 class MockTransport:
     def __init__(self, sockname=None, peername=None, sslcontext=False):
@@ -651,3 +657,14 @@ def app(scope):
     protocol.data_received(UPGRADE_REQUEST)
     assert b"HTTP/1.1 400 Bad Request" in protocol.transport.buffer
     assert b"Missing Sec-WebSocket-Version header" in protocol.transport.buffer
+
+
+@pytest.mark.parametrize("protocol_cls", [HttpToolsProtocol, H11Protocol])
+def test_invalid_http_request(protocol_cls):
+    def app(scope):
+        return Response("Hello, world", media_type="text/plain")
+
+    protocol = get_connected_protocol(app, protocol_cls)
+    protocol.data_received(INVALID_REQUEST)
+    assert b"HTTP/1.1 400 Bad Request" in protocol.transport.buffer
+    assert b"Invalid HTTP request received." in protocol.transport.buffer
diff --git a/uvicorn/protocols/http/h11_impl.py b/uvicorn/protocols/http/h11_impl.py
index 91269d735..a80458519 100644
--- a/uvicorn/protocols/http/h11_impl.py
+++ b/uvicorn/protocols/http/h11_impl.py
@@ -179,7 +179,7 @@ def handle_events(self):
             except h11.RemoteProtocolError as exc:
                 msg = "Invalid HTTP request received."
                 self.logger.warning(msg)
-                self.transport.close()
+                self.send_400_response(msg)
                 return
             event_type = type(event)
 
@@ -268,21 +268,7 @@ def handle_upgrade(self, event):
         if upgrade_value != b'websocket' or self.ws_protocol_class is None:
             msg = "Unsupported upgrade request."
             self.logger.warning(msg)
-            reason = STATUS_PHRASES[400]
-            headers = [
-                (b"content-type", b"text/plain; charset=utf-8"),
-                (b"connection", b"close"),
-            ]
-            event = h11.Response(status_code=400, headers=headers, reason=reason)
-            output = self.conn.send(event)
-            self.transport.write(output)
-            event = h11.Data(data=b'Unsupported upgrade request.')
-            output = self.conn.send(event)
-            self.transport.write(output)
-            event = h11.EndOfMessage()
-            output = self.conn.send(event)
-            self.transport.write(output)
-            self.transport.close()
+            self.send_400_response(msg)
             return
 
         self.connections.discard(self)
@@ -301,6 +287,23 @@ def handle_upgrade(self, event):
         protocol.data_received(b''.join(output))
         self.transport.set_protocol(protocol)
 
+    def send_400_response(self, msg):
+        reason = STATUS_PHRASES[400]
+        headers = [
+            (b"content-type", b"text/plain; charset=utf-8"),
+            (b"connection", b"close"),
+        ]
+        event = h11.Response(status_code=400, headers=headers, reason=reason)
+        output = self.conn.send(event)
+        self.transport.write(output)
+        event = h11.Data(data=bytes(msg, 'utf-8'))
+        output = self.conn.send(event)
+        self.transport.write(output)
+        event = h11.EndOfMessage()
+        output = self.conn.send(event)
+        self.transport.write(output)
+        self.transport.close()
+
     def on_response_complete(self):
         self.state["total_requests"] += 1
 
diff --git a/uvicorn/protocols/http/httptools_impl.py b/uvicorn/protocols/http/httptools_impl.py
index bfffe9ede..39423d1bb 100644
--- a/uvicorn/protocols/http/httptools_impl.py
+++ b/uvicorn/protocols/http/httptools_impl.py
@@ -169,7 +169,8 @@ def data_received(self, data):
         except httptools.parser.errors.HttpParserError as exc:
             msg = "Invalid HTTP request received."
             self.logger.warning(msg)
-            self.transport.close()
+            self.send_400_response(msg)
+            return
         except httptools.HttpParserUpgrade as exc:
             self.handle_upgrade()
 
@@ -182,16 +183,7 @@ def handle_upgrade(self):
         if upgrade_value != b'websocket' or self.ws_protocol_class is None:
             msg = "Unsupported upgrade request."
             self.logger.warning(msg)
-            content = [STATUS_LINE[400], DEFAULT_HEADERS]
-            content.extend([
-                b"content-type: text/plain; charset=utf-8\r\n",
-                b"content-length: " + str(len(msg)).encode('ascii') + b"\r\n",
-                b"connection: close\r\n",
-                b"\r\n",
-                msg.encode('ascii')
-            ])
-            self.transport.write(b"".join(content))
-            self.transport.close()
+            self.send_400_response(msg)
             return
 
         self.connections.discard(self)
@@ -211,6 +203,18 @@ def handle_upgrade(self):
         protocol.data_received(b''.join(output))
         self.transport.set_protocol(protocol)
 
+    def send_400_response(self, msg):
+        content = [STATUS_LINE[400], DEFAULT_HEADERS]
+        content.extend([
+            b"content-type: text/plain; charset=utf-8\r\n",
+            b"content-length: " + str(len(msg)).encode('ascii') + b"\r\n",
+            b"connection: close\r\n",
+            b"\r\n",
+            msg.encode('ascii')
+        ])
+        self.transport.write(b"".join(content))
+        self.transport.close()
+
     # Parser callbacks
     def on_url(self, url):
         method = self.parser.get_method()