diff --git a/google/cloud/pubsub_v1/subscriber/scheduler.py b/google/cloud/pubsub_v1/subscriber/scheduler.py
index a11ca490b..b8f2b592c 100644
--- a/google/cloud/pubsub_v1/subscriber/scheduler.py
+++ b/google/cloud/pubsub_v1/subscriber/scheduler.py
@@ -21,6 +21,7 @@
 import abc
 import concurrent.futures
 import queue
+import warnings
 
 
 class Scheduler(metaclass=abc.ABCMeta):
@@ -114,7 +115,14 @@ def schedule(self, callback, *args, **kwargs):
         Returns:
             None
         """
-        self._executor.submit(callback, *args, **kwargs)
+        try:
+            self._executor.submit(callback, *args, **kwargs)
+        except RuntimeError:
+            warnings.warn(
+                "Scheduling a callback after executor shutdown.",
+                category=RuntimeWarning,
+                stacklevel=2,
+            )
 
     def shutdown(self, await_msg_callbacks=False):
         """Shut down the scheduler and immediately end all pending callbacks.
@@ -142,6 +150,8 @@ def shutdown(self, await_msg_callbacks=False):
         try:
             while True:
                 work_item = self._executor._work_queue.get(block=False)
+                if work_item is None:  # Exceutor in shutdown mode.
+                    continue
                 dropped_messages.append(work_item.args[0])
         except queue.Empty:
             pass
diff --git a/tests/unit/pubsub_v1/subscriber/test_scheduler.py b/tests/unit/pubsub_v1/subscriber/test_scheduler.py
index 82a6719d7..0545c967c 100644
--- a/tests/unit/pubsub_v1/subscriber/test_scheduler.py
+++ b/tests/unit/pubsub_v1/subscriber/test_scheduler.py
@@ -16,6 +16,7 @@
 import queue
 import threading
 import time
+import warnings
 
 import mock
 
@@ -61,6 +62,24 @@ def callback(*args, **kwargs):
     assert sorted(called_with) == expected_calls
 
 
+def test_schedule_after_executor_shutdown_warning():
+    def callback(*args, **kwargs):
+        pass
+
+    scheduler_ = scheduler.ThreadScheduler()
+
+    scheduler_.schedule(callback, "arg1", kwarg1="meep")
+    scheduler_._executor.shutdown()
+
+    with warnings.catch_warnings(record=True) as warned:
+        scheduler_.schedule(callback, "arg2", kwarg2="boop")
+
+    assert len(warned) == 1
+    assert issubclass(warned[0].category, RuntimeWarning)
+    warning_msg = str(warned[0].message)
+    assert "after executor shutdown" in warning_msg
+
+
 def test_shutdown_nonblocking_by_default():
     called_with = []
     at_least_one_called = threading.Event()
@@ -125,3 +144,30 @@ def callback(message):
 
     err_msg = "Shutdown did not wait for the already running callbacks to complete."
     assert at_least_one_completed.is_set(), err_msg
+
+
+def test_shutdown_handles_executor_queue_sentinels():
+    at_least_one_called = threading.Event()
+
+    def callback(_):
+        at_least_one_called.set()
+        time.sleep(1.0)
+
+    executor = concurrent.futures.ThreadPoolExecutor(max_workers=1)
+    scheduler_ = scheduler.ThreadScheduler(executor=executor)
+
+    scheduler_.schedule(callback, "message_1")
+    scheduler_.schedule(callback, "message_2")
+    scheduler_.schedule(callback, "message_3")
+
+    # Simulate executor shutdown from another thread.
+    executor._work_queue.put(None)
+    executor._work_queue.put(None)
+
+    at_least_one_called.wait()
+    dropped = scheduler_.shutdown(await_msg_callbacks=True)
+
+    assert len(set(dropped)) == 2  # Also test for item uniqueness.
+    for msg in dropped:
+        assert msg is not None
+        assert msg.startswith("message_")