diff --git a/socketio/base_manager.py b/socketio/base_manager.py
index 795bb93..936101a 100644
--- a/socketio/base_manager.py
+++ b/socketio/base_manager.py
@@ -103,6 +103,8 @@ class BaseManager(object):
 
     def enter_room(self, sid, namespace, room, eio_sid=None):
         """Add a client to a room."""
+        if eio_sid is None and namespace not in self.rooms:
+            raise ValueError('sid is not connected to requested namespace')
         if namespace not in self.rooms:
             self.rooms[namespace] = {}
         if room not in self.rooms[namespace]:
diff --git a/tests/common/test_base_manager.py b/tests/common/test_base_manager.py
index af949ce..a695fed 100644
--- a/tests/common/test_base_manager.py
+++ b/tests/common/test_base_manager.py
@@ -1,6 +1,8 @@
 import unittest
 from unittest import mock
 
+import pytest
+
 from socketio import base_manager
 
 
@@ -119,6 +121,11 @@ class TestBaseManager(unittest.TestCase):
         self.bm.connect('123', '/foo')
         self.bm.disconnect('123', '/bar')  # should not assert
 
+    def test_enter_room_bad_namespace(self):
+        sid = self.bm.connect('123', '/')
+        with pytest.raises(ValueError):
+            self.bm.enter_room(sid, '/foo', 'bar')
+
     def test_trigger_callback(self):
         sid1 = self.bm.connect('123', '/')
         sid2 = self.bm.connect('123', '/foo')