Undestr also affecting vehicles

I have a zone with undestr enabled. The documentation for this plugin says when this flag is enabled, buildings will not take damage.
However, this seems to also affect vehicles. For example, it is impossible to destory a heli, or Bradley at the launch site, etc. I want the undestr to only affect buildings. How do I do this?

added this to OnEntityTakeDamage to allow bradley to take damage in an undestr zone

if (entity is BradleyAPC)
{
	return null;
}​