It’s apparently trying to resolve the route within the Discourse Ember app, which doesn’t work because the upgrader isn’t actually part of the app. It needs to perform a browser-level page navigation.
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Fixed via:
https://github.com/discourse/discourse/commit/585fc26f8ee66b2fd5b442b699bfb6954181a21b
Thanks for reporting this issue @notriddle. 
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