diff --git a/notebook/static/notebook/js/notebook.js b/notebook/static/notebook/js/notebook.js
index 9f0404c79..58b2d7c2e 100644
--- a/notebook/static/notebook/js/notebook.js
+++ b/notebook/static/notebook/js/notebook.js
@@ -2073,7 +2073,7 @@ import {ShortcutEditor} from 'notebook/js/shortcuteditor';
         var success = $.proxy(this._session_started, this);
         var failure = $.proxy(this._session_start_failed, this);
 
-        if (this.session !== null) {
+        if (this.session && this.session.kernel) {
             this.session.restart(options, success, failure);
         } else {
             this.session = new session.Session(options);