Fixes #15970.

Change-Id: I3f7d8316069a69d0e3859aaa96bc1414487fead0
Reviewed-on: https://go-review.googlesource.com/23921
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>