diff --git a/plover/machine/keymap.py b/plover/machine/keymap.py
index 3db5f419b..9fbba16bf 100644
--- a/plover/machine/keymap.py
+++ b/plover/machine/keymap.py
@@ -85,7 +85,7 @@ def get_action(self, key, default=None):
     def keys_to_actions(self, key_list):
         action_list = []
         for key in key_list:
-            assert key in self._keys
+            assert key in self._keys, "'%s' not in %s" % (key, self._keys)
             action = self._bindings[key]
             if 'no-op' != action:
                 action_list.append(action)
diff --git a/plover/machine/procat.py b/plover/machine/procat.py
index 06797a10d..59a48efd2 100644
--- a/plover/machine/procat.py
+++ b/plover/machine/procat.py
@@ -57,7 +57,9 @@ def process_steno_packet(raw):
         for i, b in enumerate(iterbytes(raw[:3])):
             for j in range(0, 8):
                 if b & 0x80 >> j:
-                    steno_keys.append(STENO_KEY_CHART[i * 8 + j])
+                    key = STENO_KEY_CHART[i * 8 + j]
+                    if key:
+                        steno_keys.append(key)
         return steno_keys