Fixes golang/go#21358

Change-Id: I8f8653df3c1e3c86cc8515a78509a8af03160944
Reviewed-on: https://go-review.googlesource.com/c/149277
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>