Index: ps/trunk/binaries/data/mods/public/simulation/ai/common-api/entity.js
===================================================================
--- ps/trunk/binaries/data/mods/public/simulation/ai/common-api/entity.js
+++ ps/trunk/binaries/data/mods/public/simulation/ai/common-api/entity.js
@@ -743,6 +743,10 @@
 		return undefined;
 	},
 
+	"garrisonHolderID": function() {
+		return this._entity.garrisonHolderID;
+	},
+
 	"garrisoned": function() { return this._entity.garrisoned; },
 
 	"garrisonedSlots": function() {
Index: ps/trunk/binaries/data/mods/public/simulation/ai/petra/headquarters.js
===================================================================
--- ps/trunk/binaries/data/mods/public/simulation/ai/petra/headquarters.js
+++ ps/trunk/binaries/data/mods/public/simulation/ai/petra/headquarters.js
@@ -398,8 +398,7 @@
 			if (!ent.position())
 			{
 				// we are autogarrisoned, check that the holder is registered in the garrisonManager
-				let holderId = ent.unitAIOrderData()[0].target;
-				let holder = gameState.getEntityById(holderId);
+				let holder = gameState.getEntityById(ent.garrisonHolderID());
 				if (holder)
 					this.garrisonManager.registerHolder(gameState, holder);
 			}
Index: ps/trunk/binaries/data/mods/public/simulation/components/AIProxy.js
===================================================================
--- ps/trunk/binaries/data/mods/public/simulation/components/AIProxy.js
+++ ps/trunk/binaries/data/mods/public/simulation/components/AIProxy.js
@@ -106,6 +106,13 @@
 	this.changes.hitpoints = msg.to;
 };
 
+AIProxy.prototype.OnGarrisonedStateChanged = function(msg)
+{
+	if (!this.NotifyChange())
+		return;
+	this.changes.garrisonHolderID = msg.holderID;
+};
+
 AIProxy.prototype.OnCapturePointsChanged = function(msg)
 {
 	if (!this.NotifyChange())
@@ -327,6 +334,13 @@
 		ret.garrisoned = cmpGarrisonHolder.GetEntities();
 	}
 
+	let cmpGarrisonable = Engine.QueryInterface(this.entity, IID_Garrisonable);
+	if (cmpGarrisonable)
+	{
+		// Updated by OnGarrisonedStateChanged
+		ret.garrisonHolderID = cmpGarrisonable.HolderID();
+	}
+
 	let cmpTerritoryDecay = Engine.QueryInterface(this.entity, IID_TerritoryDecay);
 	if (cmpTerritoryDecay)
 		ret.decaying = cmpTerritoryDecay.IsDecaying();
Index: ps/trunk/binaries/data/mods/public/simulation/components/Garrisonable.js
===================================================================
--- ps/trunk/binaries/data/mods/public/simulation/components/Garrisonable.js
+++ ps/trunk/binaries/data/mods/public/simulation/components/Garrisonable.js
@@ -88,6 +88,10 @@
 	if (cmpPosition)
 		cmpPosition.MoveOutOfWorld();
 
+	Engine.PostMessage(this.entity, MT_GarrisonedStateChanged, {
+		"holderID": target
+	});
+
 	if (renamed)
 		return true;
 
@@ -161,6 +165,10 @@
 	if (cmpAura && cmpAura.HasGarrisonAura())
 		cmpAura.RemoveGarrisonAura(this.holder);
 
+	Engine.PostMessage(this.entity, MT_GarrisonedStateChanged, {
+		"holderID": INVALID_ENTITY
+	});
+
 	if (renamed)
 		return true;
 
Index: ps/trunk/binaries/data/mods/public/simulation/components/interfaces/Garrisonable.js
===================================================================
--- ps/trunk/binaries/data/mods/public/simulation/components/interfaces/Garrisonable.js
+++ ps/trunk/binaries/data/mods/public/simulation/components/interfaces/Garrisonable.js
@@ -1 +1,7 @@
 Engine.RegisterInterface("Garrisonable");
+
+/**
+ * Message of the form { "holderID": number }
+ * sent from the Garrisonable component whenever the garrisoned state changes.
+ */
+Engine.RegisterMessageType("GarrisonedStateChanged");