Cool. Sorry for spamming this thread with that question. Appreciate the quick reply!
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Wasn't really sure where to put this. Playing Storm King's Thunder. Player decided to wear the Dragon guard Breastplate, can't remember where in the module they picked this up. When equipped it did nothing to AC so I went in and manually set it. Anyway tonight player was not using shield so I told him to unequip it in inventory and doing so change his AC Base to -10. I'm guessing this item is the issue and if I dragged a regular breastplate on the character this wouldn't happen but thought I would mention it.
Mot sure where you got it from either since such a thing doesn't exist in Storm King's Thunder. It does in Lost Mines of Phandelver though so maybe that's where it came from. At any rate there is an issue with this item at the moment (in LMoP) but it has been fixed in v3.2.2. which should be released shortly.
For future reference the place to report 5e specific bugs is in the 5e bugs thread stickied at the top of the 5e forum.