Some Teams Immune To NPC Melee Damage

[LostInTheWired]

Tracking down a weird issue where zombies/antlions weren't doing damage to players, it turns out that teams 1-4 (classes in our case), do not take damage. But team 5, and any team we define above that, do. We're honestly stumped as to why that would be. We have nothing in any damage hooks that would do that, all the teams are defined exactly the same then never touched again, other than setting players to a team, or reading a player's team. Seriously, our team code is as basic as this:
[LUA]TEAM_WASTELANDER = 1
TEAM_SCAVENGER = 2
TEAM_SCIENCE = 3
TEAM_ENGINEER = 4
TEAM_CULTIVATOR = 5

team.SetUp( TEAM_WASTELANDER, "Wastelander", Color( 125, 125, 125, 255 ) ) --Gray
team.SetUp( TEAM_SCAVENGER, "Scavenger", Color( 102, 51, 0, 225 ) ) --Brown
team.SetUp( TEAM_SCIENCE, "Science", Color( 0, 0, 153, 225 ) ) --Blue
team.SetUp( TEAM_ENGINEER, "Engineer", Color( 255, 204, 0, 225 ) ) --Orange
team.SetUp( TEAM_CULTIVATOR, "Cultivator", Color( 51, 153, 0, 225 ) ) --Green[/LUA]

So, what do you guys think is going on? It just doesn't make sense to us.