diff --git a/.gitignore b/.gitignore
index 3c1a62c..679bf0f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@ __pycache__
 .eggs
 /dist/
 /wheelhouse/
+.DS_Store
\ No newline at end of file
diff --git a/portaudio b/portaudio
index 147dd72..7c45fed 160000
--- a/portaudio
+++ b/portaudio
@@ -1 +1 @@
-Subproject commit 147dd722548358763a8b649b3e4b41dfffbcfbb6
+Subproject commit 7c45fed9322bd86b5dd0da4547b15d9ac3b9b0ac
diff --git a/src/rtmixer.c b/src/rtmixer.c
index 9ac3133..6506220 100644
--- a/src/rtmixer.c
+++ b/src/rtmixer.c
@@ -128,7 +128,10 @@ int callback(const void* input, void* output, frame_t frameCount
   struct state* state = userData;
   CALLBACK_ASSERT(state);
 
-  memset(output, 0, sizeof(float) * state->output_channels * frameCount);
+  if (output)  // NULL for input-only streams
+  {
+    memset(output, 0, sizeof(float) * state->output_channels * frameCount);
+  }
 
   get_stats(frameCount, statusFlags, &(state->stats));
 
diff --git a/src/rtmixer.py b/src/rtmixer.py
index 0cdde95..705127b 100644
--- a/src/rtmixer.py
+++ b/src/rtmixer.py
@@ -41,6 +41,15 @@ def __init__(self, kind, qsize=16, **kwargs):
         self._actions = {}
         self._temp_action_ptr = _ffi.new('struct action**')
 
+    def __del__(self):
+        # The C callback uses memory owned by this object, so the stream
+        # must be closed before that memory is freed.
+        try:
+            self.abort()
+            self.close()
+        except Exception:
+            pass  # e.g. __init__() failed or interpreter shutdown
+
     @property
     def actions(self):
         """The set of active "actions"."""
@@ -76,7 +85,8 @@ def cancel(self, action, time=0, allow_belated=True):
             requested_time=time,
             action=action,
         ))
-        self._enqueue(cancel_action)
+        # The callback dereferences *action*, so it must be kept alive:
+        self._enqueue(cancel_action, keep_alive=action)
         return cancel_action
 
     def fetch_and_reset_stats(self, time=0, allow_belated=True):
@@ -126,6 +136,11 @@ def _check_channels(self, channels, kind):
 
     def _enqueue(self, action, keep_alive=None):
         self._drain_result_q()
+        # Each action ends up in the result queue exactly once, so limiting
+        # the number of pending actions guarantees that it never overflows:
+        if len(self._actions) >= len(self._result_q):
+            raise RuntimeError(
+                'Too many pending actions, consider increasing qsize')
         self._temp_action_ptr[0] = action
         ret = self._action_q.write(self._temp_action_ptr)
         if ret != 1:
@@ -149,6 +164,13 @@ class Mixer(_Base):
     except *callback* (a callback function implemented in C is used
     internally) and *dtype* (which is always ``'float32'``).
 
+    The additional keyword argument *qsize* (default: 16) sets the size
+    of the queues used to communicate with the callback, which is also
+    the maximum number of pending actions, i.e. actions that have been
+    started (or scheduled) but haven't finished yet.  If an action is
+    started while *qsize* actions are pending, a `RuntimeError` is
+    raised.  It must be a power of 2.
+
     Uses default values from `sounddevice.default` (except *dtype*,
     which is always ``'float32'``).
 
@@ -217,6 +239,8 @@ class Recorder(_Base):
     except *callback* (a callback function implemented in C is used
     internally) and *dtype* (which is always ``'float32'``).
 
+    The additional keyword argument *qsize* is described in `Mixer`.
+
     Uses default values from `sounddevice.default` (except *dtype*,
     which is always ``'float32'``).
 
@@ -282,6 +306,8 @@ class MixerAndRecorder(Mixer, Recorder):
     *callback* (a callback function implemented in C is used internally)
     and *dtype* (which is always ``'float32'``).
 
+    The additional keyword argument *qsize* is described in `Mixer`.
+
     Uses default values from `sounddevice.default` (except *dtype*,
     which is always ``'float32'``).