Temporal.Duration.prototype.{round,total}: Is CreateTemporalDuration really fallible when creating from balanced valid Duration?

[anba]

Step 39.a:

Let dateDuration be ? CreateTemporalDuration(duration.[[Years]], duration.[[Months]], duration.[[Weeks]], duration.[[Days]] + targetTime.[[Days]], 0, 0, 0, 0, 0, 0).

duration is valid duration, so when can duration.[[Days]] + targetTime.[[Days]] create an invalid duration?