diff --git a/jupyter_server/services/kernels/handlers.py b/jupyter_server/services/kernels/handlers.py
index 6e4d6989f3..e84b4e7ca4 100644
--- a/jupyter_server/services/kernels/handlers.py
+++ b/jupyter_server/services/kernels/handlers.py
@@ -7,6 +7,8 @@
 import json
 import logging
 from textwrap import dedent
+from traceback import format_tb
+import traceback
 
 from ipython_genutils.py3compat import cast_unicode
 from jupyter_client import protocol_version as client_protocol_version
@@ -79,7 +81,10 @@ async def post(self, kernel_id, action):
             try:
                 await km.restart_kernel(kernel_id)
             except Exception as e:
-                self.log.error("Exception restarting kernel", exc_info=True)
+                message = "Exception restarting kernel"
+                self.log.error(message, exc_info=True)
+                traceback = format_tb(e.__traceback__)
+                self.write(json.dumps(dict(message=message, traceback=traceback)))
                 self.set_status(500)
             else:
                 model = await ensure_async(km.kernel_model(kernel_id))
@@ -326,6 +331,7 @@ async def pre_get(self):
         # We don't want to wait forever, because browsers don't take it well when
         # servers never respond to websocket connection requests.
         kernel = self.kernel_manager.get_kernel(self.kernel_id)
+        await kernel.ready
         self.session.key = kernel.session.key
         future = self.request_kernel_info()
 
@@ -446,6 +452,7 @@ def on_message(self, msg):
     def _on_zmq_reply(self, stream, msg_list):
         idents, fed_msg_list = self.session.feed_identities(msg_list)
         msg = self.session.deserialize(fed_msg_list)
+
         parent = msg["parent_header"]
 
         def write_stderr(error_message):
diff --git a/jupyter_server/services/kernels/kernelmanager.py b/jupyter_server/services/kernels/kernelmanager.py
index 02a1e4d346..074fa5b5b2 100644
--- a/jupyter_server/services/kernels/kernelmanager.py
+++ b/jupyter_server/services/kernels/kernelmanager.py
@@ -5,6 +5,7 @@
 """
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
+import asyncio
 import os
 from collections import defaultdict
 from datetime import datetime
@@ -51,6 +52,8 @@ def _default_kernel_manager_class(self):
 
     kernel_argv = List(Unicode())
 
+    use_pending_kernels = True
+
     root_dir = Unicode(config=True)
 
     _kernel_connections = Dict()
@@ -209,7 +212,7 @@ async def start_kernel(self, kernel_id=None, path=None, **kwargs):
                 kwargs["kernel_id"] = kernel_id
             kernel_id = await ensure_async(self.pinned_superclass.start_kernel(self, **kwargs))
             self._kernel_connections[kernel_id] = 0
-            self._kernel_ports[kernel_id] = self._kernels[kernel_id].ports
+            task = asyncio.create_task(self._get_ports(kernel_id))
             self.start_watching_activity(kernel_id)
             self.log.info("Kernel started: %s" % kernel_id)
             self.log.debug("Kernel args: %r" % kwargs)
@@ -233,6 +236,11 @@ async def start_kernel(self, kernel_id=None, path=None, **kwargs):
 
         return kernel_id
 
+    async def _get_ports(self, kernel_id):
+        km = self.get_kernel(kernel_id)
+        await km.ready
+        self._kernel_ports[kernel_id] = km.ports
+
     def ports_changed(self, kernel_id):
         """Used by ZMQChannelsHandler to determine how to coordinate nudge and replays.