diff --git a/portal/contextlib.py b/portal/contextlib.py
index c427c1b..f5100db 100644
--- a/portal/contextlib.py
+++ b/portal/contextlib.py
@@ -4,6 +4,7 @@
 import sys
 import threading
 import traceback
+import warnings
 
 import cloudpickle
 import psutil
@@ -143,7 +144,12 @@ def add_worker(self, worker):
       return
     if hasattr(worker, 'thread'):
       assert current != worker.thread
-    current.children.append(worker)
+    try:
+      current.children.append(worker)
+    except AttributeError:
+      warnings.warn(
+          'Using Portal from plain Python threads is discouraged because ' +
+          'they can cause hangs during shutdown.')
 
   def children(self, thread):
     current = thread or threading.current_thread()
diff --git a/tests/test_process.py b/tests/test_process.py
index 68db62a..7c93a0d 100644
--- a/tests/test_process.py
+++ b/tests/test_process.py
@@ -52,7 +52,7 @@ def fn():
     worker = portal.Process(fn, start=True)
     worker.kill()
     assert not worker.running
-    assert worker.exitcode < 0
+    assert abs(worker.exitcode) >= 1
 
   @pytest.mark.parametrize('repeat', range(5))
   def test_kill_with_subproc(self, repeat):
@@ -80,7 +80,7 @@ def inner(ready, queue):
     ready.wait()
     worker.kill()
     assert not worker.running
-    assert worker.exitcode < 0
+    assert abs(worker.exitcode) >= 1
     assert not portal.proc_alive(queue.get())
     assert not portal.proc_alive(queue.get())
 
@@ -99,7 +99,7 @@ def inner(ready):
     ready.wait()
     worker.kill()
     assert not worker.running
-    assert worker.exitcode < 0
+    assert abs(worker.exitcode) >= 1
 
   def test_initfn(self):
 
diff --git a/tests/test_server.py b/tests/test_server.py
index 32be753..f0a2357 100644
--- a/tests/test_server.py
+++ b/tests/test_server.py
@@ -199,7 +199,7 @@ def postfn(x):
   def test_shared_pool(self, repeat, Server):
 
     def slow(x):
-      time.sleep(0.2)
+      time.sleep(0.5)
       return x
 
     def fast(x):
@@ -226,7 +226,7 @@ def fast(x):
   def test_separate_pools(self, repeat, Server):
 
     def slow(x):
-      time.sleep(0.1)
+      time.sleep(0.5)
       return x
 
     def fast(x):
diff --git a/tests/test_thread.py b/tests/test_thread.py
index 1347285..357463c 100644
--- a/tests/test_thread.py
+++ b/tests/test_thread.py
@@ -91,4 +91,4 @@ def inner(ready):
     assert not worker.running
     assert not proc[0].running
     assert worker.exitcode == 2
-    assert proc[0].exitcode < 0
+    assert abs(proc[0].exitcode) >= 1