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I realized after committing that this can sometimes cause array( 'until' => null ) to be passed as the query to $sk->linkKnown(). However, this doesn't actually seem to break anything or even cause warnings(!) -- the null propagates all the way to wfArrayToCGI(), which happily feeds it to urlencode(), which apparently accepts it and returns an empty string. Still, it might be worth explicitly setting it to a non-null value (say, "") rather than relying on this.