Fixes #13236

Change-Id: If902ac66718e0a0790fab9835921ce4ef980965b
Reviewed-on: https://go-review.googlesource.com/21183
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>