Fixes #6007

Change-Id: I239a1699122e086e907ac1f18b1c86a650e1438a
Reviewed-on: https://go-review.googlesource.com/81135
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>