diff --git a/cassandra/connection.py b/cassandra/connection.py
index 08501d0a2b..7d5ee0f47b 100644
--- a/cassandra/connection.py
+++ b/cassandra/connection.py
@@ -1219,15 +1219,19 @@ def send_msg(self, msg, request_id, cb, encoder=ProtocolHandler.encode_message,
         # queue the decoder function with the request
         # this allows us to inject custom functions per request to encode, decode messages
         self._requests[request_id] = (cb, decoder, result_metadata)
-        msg = encoder(msg, request_id, self.protocol_version, compressor=self.compressor,
-                      allow_beta_protocol_version=self.allow_beta_protocol_version)
+        try:
+            msg = encoder(msg, request_id, self.protocol_version, compressor=self.compressor,
+                          allow_beta_protocol_version=self.allow_beta_protocol_version)
 
-        if self._is_checksumming_enabled:
-            buffer = io.BytesIO()
-            self._segment_codec.encode(buffer, msg)
-            msg = buffer.getvalue()
+            if self._is_checksumming_enabled:
+                buffer = io.BytesIO()
+                self._segment_codec.encode(buffer, msg)
+                msg = buffer.getvalue()
 
-        self.push(msg)
+            self.push(msg)
+        except Exception:
+            self._requests.pop(request_id, None)
+            raise
         return len(msg)
 
     def wait_for_response(self, msg, timeout=None, **kwargs):
@@ -1262,9 +1266,16 @@ def wait_for_responses(self, *msgs, **kwargs):
                 self.in_flight += available
 
             for i, request_id in enumerate(request_ids):
-                self.send_msg(msgs[messages_sent + i],
-                              request_id,
-                              partial(waiter.got_response, index=messages_sent + i))
+                try:
+                    self.send_msg(msgs[messages_sent + i],
+                                  request_id,
+                                  partial(waiter.got_response, index=messages_sent + i))
+                except Exception:
+                    unsent_request_ids = request_ids[i:]
+                    with self.lock:
+                        self.in_flight -= len(unsent_request_ids)
+                        self.request_ids.extend(unsent_request_ids)
+                    raise
             messages_sent += available
 
             if messages_sent == len(msgs):
diff --git a/tests/unit/test_connection.py b/tests/unit/test_connection.py
index 2fa7c71196..5e2d57192e 100644
--- a/tests/unit/test_connection.py
+++ b/tests/unit/test_connection.py
@@ -18,14 +18,14 @@
 from threading import Lock
 from unittest.mock import Mock, ANY, call, patch
 
-from cassandra import OperationTimedOut
+from cassandra import ConsistencyLevel, OperationTimedOut
 from cassandra.cluster import Cluster
 from cassandra.connection import (Connection, HEADER_DIRECTION_TO_CLIENT, ProtocolError,
                                   locally_supported_compressions, ConnectionHeartbeat, _Frame, Timer, TimerManager,
-                                  ConnectionException, ConnectionShutdown, DefaultEndPoint, ShardAwarePortGenerator)
+                                  ConnectionException, ConnectionShutdown, ConnectionBusy, DefaultEndPoint, ShardAwarePortGenerator)
 from cassandra.marshal import uint8_pack, uint32_pack, int32_pack
 from cassandra.protocol import (write_stringmultimap, write_int, write_string,
-                                SupportedMessage, ProtocolHandler, ResultMessage,
+                                SupportedMessage, ProtocolHandler, ResultMessage, QueryMessage,
                                 RESULT_KIND_SET_KEYSPACE)
 
 from tests.util import wait_until, assertRegex
@@ -363,6 +363,31 @@ def test_wait_for_responses_shutdown_includes_last_error(self):
         assert "already closed" in error_message
         assert "Bad file descriptor" in error_message
 
+    def test_wait_for_responses_releases_request_id_when_send_fails(self):
+        c = self.make_connection()
+        c._socket_writable = False
+        initial_in_flight = c.in_flight
+        initial_request_ids = len(c.request_ids)
+
+        with pytest.raises(ConnectionBusy):
+            c.wait_for_responses(Mock())
+
+        assert c.in_flight == initial_in_flight
+        assert len(c.request_ids) == initial_request_ids
+        assert not c._requests
+
+    def test_wait_for_responses_releases_request_id_when_send_raises_after_registration(self):
+        c = self.make_connection()
+        c.push = Mock(side_effect=ConnectionException("write failed"))
+        initial_in_flight = c.in_flight
+        initial_request_ids = len(c.request_ids)
+
+        with pytest.raises(ConnectionException):
+            c.wait_for_responses(QueryMessage("SELECT * FROM system.local", ConsistencyLevel.ONE))
+
+        assert c.in_flight == initial_in_flight
+        assert len(c.request_ids) == initial_request_ids
+        assert not c._requests
 
 @patch('cassandra.connection.ConnectionHeartbeat._raise_if_stopped')
 class ConnectionHeartbeatTest(unittest.TestCase):