Steps to Reproduce:

1. Go to Internet Explorer 11 at Windows 7 or 8 (or 10 when using the IE mode in MS Edge).
2. When logged-in, go to your Preferences settings page, or click "Beta".
3. If "Preferences" is clicked, then click "Beta features" tab
4. Enable "New wikitext mode".
5. If "Temporarily disable the visual editor while it is in beta" option is checkmarked, then de-checkmark the option to re-enable the selection menu.
6. Under the selection menu, select "Always give me the source editor".
7. When settings are saved, go to any page, like a sandbox or an article.
8. Click the "Edit" tab.

Actual Results:

• The URL address bar says "https://en.wikipedia.org/wiki/undefined?action=edit"
• When at any subpage of a page and using steps to reproduce, the URL address bar says https://en.wikipedia.org/wiki/<article/page>/undefined?action=edit, e.g. https://en.wikipedia.org/wiki/Wikipedia:WikiProject_Cricket/undefined?action=edit
• When "Article" tab is clicked to exit the NWE, the address bar still retains the above URL.
• When a page is refreshed/reloaded (i.e. press F5), the browser directs a user to (literally) the disambiguation page about "Undefined".

Expected Results:

• In Chrome and Firefox, the URL address bar still remains as https://en.wikipedia.org/wiki/<article/page>?action=edit.
• When either exiting the NWE or refreshing a page, the browser should not redirect a user to that disambiguation page.
• A user at IE11 should not have experienced the "Actual Results" described above.