Steps to Reproduce:

1. Log in as an admin on enwiki
2. Navigate to https://en.wikipedia.org/wiki/Special:Undelete/User:RoySmith/sandbox/del-test
3. Click any of the active diff links in the "Page history" section

Alternatively, skip the last two steps and go directly to:

https://en.wikipedia.org/w/index.php?title=Special:Undelete&target=User%3ARoySmith%2Fsandbox%2Fdel-test&timestamp=20191109142521&diff=prev

Actual Results:
You get to a page which says:

Internal error
[XcbNGQpAICAAAJ8dNJcAAACO] 2019-11-09 14:28:41: Fatal exception of type "InvalidArgumentException"

Expected Results:
Not an internal error.