diff --git a/IPython/frontend/html/notebook/notebookapp.py b/IPython/frontend/html/notebook/notebookapp.py
index 1c45c2f..caf3a38 100644
--- a/IPython/frontend/html/notebook/notebookapp.py
+++ b/IPython/frontend/html/notebook/notebookapp.py
@@ -142,7 +142,8 @@ class NotebookWebApplication(web.Application):
# TODO: This causes a hard crash in ZMQStream.close, which sets
# self.socket to None to hastily. We will need to fix this in PyZMQ
# itself. For now, we just leave the old kernel running :(
- # self.kill_kernel(kernel_id)
+ # Maybe this is fixed now, but nothing was changed really.
+ self.kill_kernel(kernel_id)
self.log.debug("Kernel restarted: %s -> %s" % (kernel_id, new_kernel_id))
return new_kernel_id