diff --git a/OpenRA.Mods.Common/Activities/RepairBuilding.cs b/OpenRA.Mods.Common/Activities/RepairBuilding.cs
index b5c64368f7..85e50aa1e9 100644
--- a/OpenRA.Mods.Common/Activities/RepairBuilding.cs
+++ b/OpenRA.Mods.Common/Activities/RepairBuilding.cs
@@ -18,11 +18,13 @@ namespace OpenRA.Mods.Common.Activities
 	{
 		readonly Actor target;
 		readonly Health health;
+		readonly Stance validStances;
 
-		public RepairBuilding(Actor self, Actor target, EnterBehaviour enterBehaviour)
+		public RepairBuilding(Actor self, Actor target, EnterBehaviour enterBehaviour, Stance validStances)
 			: base(self, target, enterBehaviour)
 		{
 			this.target = target;
+			this.validStances = validStances;
 			health = target.Trait<Health>();
 		}
 
@@ -33,6 +35,10 @@ namespace OpenRA.Mods.Common.Activities
 
 		protected override void OnInside(Actor self)
 		{
+			var stance = self.Owner.Stances[target.Owner];
+			if (!stance.HasStance(validStances))
+				return;
+
 			if (health.DamageState == DamageState.Undamaged)
 				return;
 
diff --git a/OpenRA.Mods.Common/Traits/EngineerRepair.cs b/OpenRA.Mods.Common/Traits/EngineerRepair.cs
index 11239af792..d7acd7a136 100644
--- a/OpenRA.Mods.Common/Traits/EngineerRepair.cs
+++ b/OpenRA.Mods.Common/Traits/EngineerRepair.cs
@@ -26,6 +26,9 @@ namespace OpenRA.Mods.Common.Traits
 			"Possible values are Exit, Suicide, Dispose.")]
 		public readonly EnterBehaviour EnterBehaviour = EnterBehaviour.Dispose;
 
+		[Desc("What diplomatic stances allow target to be repaired by this actor.")]
+		public readonly Stance ValidStances = Stance.Ally;
+
 		public object Create(ActorInitializer init) { return new EngineerRepair(init, this); }
 	}
 
@@ -96,7 +99,7 @@ namespace OpenRA.Mods.Common.Traits
 				self.CancelActivity();
 
 			self.SetTargetLine(target, Color.Yellow);
-			self.QueueActivity(new RepairBuilding(self, target.Actor, info.EnterBehaviour));
+			self.QueueActivity(new RepairBuilding(self, target.Actor, info.EnterBehaviour, info.ValidStances));
 		}
 
 		class EngineerRepairOrderTargeter : UnitOrderTargeter