No test because at present it is never called in a way that fails.
When #22114 is implemented, failure will be possible. Not including this
change in that work because this change is separable and clearly correct.

Updates #22114

Change-Id: I81eb9eec8800e8082d918c0e5fb71282f538267e
Reviewed-on: https://go-review.googlesource.com/71751
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Joe Tsai <joetsai@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>