Fixes #15146.

Change-Id: I229611b9cc995a1391681c492c4d742195c787ea
Reviewed-on: https://go-review.googlesource.com/30943
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>