Fixes golang/go#29858

Change-Id: I7bd8357a7def7daab1fd6357a888ff4e676a1bee
GitHub-Last-Rev: a6dc776fa3aad595643943dcff88efcf152d6be1
GitHub-Pull-Request: golang/net#32
Reviewed-on: https://go-review.googlesource.com/c/163863Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>