"Vacuum protection requires fusion engine" neither true nor applicable to environmental sealing

[assaultdoor]

Using MML 0.47.4 on OS X, I've got an ICE-powered tank with environmental sealing. MML marks it invalid because, according to Unit Validation > Validate Current Unit, "Vacuum protection requires fusion engine." According to TO page 303, vacuum protection requires either a fission, fusion, or fuel cell engine. Also, according to the same page, environmental sealing itself doesn't require any of those engines, and allows ICE tanks to operate in tainted and toxic environments.