diff --git a/socketio/base_manager.py b/socketio/base_manager.py
index 283db13..09bef7c 100644
--- a/socketio/base_manager.py
+++ b/socketio/base_manager.py
@@ -64,6 +64,8 @@ class BaseManager(object):
 
     def disconnect(self, sid, namespace):
         """Register a client disconnect from a namespace."""
+        if namespace not in self.rooms:
+            return
         rooms = []
         for room_name, room in six.iteritems(self.rooms[namespace]):
             if sid in room:
diff --git a/tests/test_base_manager.py b/tests/test_base_manager.py
index 55141d4..d4291f9 100644
--- a/tests/test_base_manager.py
+++ b/tests/test_base_manager.py
@@ -99,6 +99,11 @@ class TestBaseManager(unittest.TestCase):
         self.bm.disconnect('123', '/')
         self.assertNotIn('123', self.bm.callbacks)
 
+    def test_disconnect_bad_namespace(self):
+        self.bm.connect('123', '/')
+        self.bm.connect('123', '/foo')
+        self.bm.disconnect('123', '/bar')  # should not assert
+
     def test_trigger_callback(self):
         self.bm.connect('123', '/')
         self.bm.connect('123', '/foo')