Fixes #18933.

Change-Id: I1ab524fdca006100ec6af572065b496f68d6a5c3
Reviewed-on: https://go-review.googlesource.com/36413
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>