Simple document doesn't compile: "Limit controls must follow a math operator"

[Christopher Barrington-Leigh]

The attached lyx document fails to compile for me, with a "Limit controls must follow a math operator" error.
In other words, the \sideset option, available in the math tools, fails.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: lyx 2.1.4-2
ProcVersionSignature: Ubuntu 4.4.0-28.47-generic 4.4.13
Uname: Linux 4.4.0-28-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Jul 10 21:58:18 2016
EcryptfsInUse: Yes
InstallationDate: Installed on 2016-02-12 (149 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
SourcePackage: lyx
UpgradeStatus: Upgraded to xenial on 2016-05-24 (47 days ago)

[Jean-Marc Lasgouttes (jmarc)]

\sideset (from amsmath package) is only designed to add sub/supercripts on both sides of big operators like \sum. The fact that it breaks in other situations could be see as an amsmath bug. Of course, LyX could have a UI to avoid that, but it is difficult to imagine which one.

[Jean-Marc Lasgouttes (jmarc)]

I al tempted to resolve as invalid (aka not our bug), but you may want to propose something else.

[Christopher Barrington-Leigh (cpbl)]

I know nothing about these classifications. I would choose "won't fix" for this kind of thing, since it is LyX's (fair) choice to support GUI constructs which won't compile in LaTeX, rather than an invalid complaint. (Like Churchill's democracy, LaTeX sucks but it's better than anything else).
However, launchpad won't let me choose "won't fix".
I don't much care what label it gets.
Thanks!