Fixes #18381.

Change-Id: I0a476cd7f6182c8d4646628477c56c133d5671ee
Reviewed-on: https://go-review.googlesource.com/34667
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>