diff --git a/OpenRa.Game/Actor.cs b/OpenRa.Game/Actor.cs
index e91f977190..3ceeca8b09 100755
--- a/OpenRa.Game/Actor.cs
+++ b/OpenRa.Game/Actor.cs
@@ -48,17 +48,23 @@ namespace OpenRa.Game
 
 		public void Tick()
 		{
-			var nextActivity = currentActivity;
-			while( nextActivity != null )
+			
+			while (currentActivity != null)
 			{
-				currentActivity = nextActivity;
-				nextActivity = nextActivity.Tick( this );
+				var a = currentActivity;
+				currentActivity = a.Tick(this) ?? new Idle();
+				if (a == currentActivity) break;
 			}
 
 			foreach (var tick in traits.WithInterface<ITick>())
 				tick.Tick(this);
 		}
 
+		public bool IsIdle
+		{
+			get { return currentActivity == null || currentActivity is Idle; }
+		}
+
 		public float2 CenterLocation;
 		public float2 SelectedSize
 		{
diff --git a/OpenRa.Game/OpenRa.Game.csproj b/OpenRa.Game/OpenRa.Game.csproj
index 818cbb205c..74d85740c5 100644
--- a/OpenRa.Game/OpenRa.Game.csproj
+++ b/OpenRa.Game/OpenRa.Game.csproj
@@ -132,6 +132,7 @@
     <Compile Include="Graphics\LineRenderer.cs" />
     <Compile Include="Graphics\OverlayRenderer.cs" />
     <Compile Include="Graphics\WorldRenderer.cs" />
+    <Compile Include="Traits\Activities\Idle.cs" />
     <Compile Include="Traits\Activities\Teleport.cs" />
     <Compile Include="BuildingInfluenceMap.cs" />
     <Compile Include="IOrderGenerator.cs" />
diff --git a/OpenRa.Game/Traits/Activities/Attack.cs b/OpenRa.Game/Traits/Activities/Attack.cs
index 5160223cb9..edb64ea905 100644
--- a/OpenRa.Game/Traits/Activities/Attack.cs
+++ b/OpenRa.Game/Traits/Activities/Attack.cs
@@ -40,7 +40,7 @@ namespace OpenRa.Game.Traits.Activities
 			var attack = self.traits.WithInterface<AttackBase>().First();
 			attack.target = Target;
 			attack.DoAttack(self);
-			return null;
+			return this;
 		}
 
 		public void Cancel(Actor self)
diff --git a/OpenRa.Game/Traits/Activities/DeliverOre.cs b/OpenRa.Game/Traits/Activities/DeliverOre.cs
index c93d7dc817..c41c69c6c5 100644
--- a/OpenRa.Game/Traits/Activities/DeliverOre.cs
+++ b/OpenRa.Game/Traits/Activities/DeliverOre.cs
@@ -59,7 +59,7 @@ namespace OpenRa.Game.Traits.Activities
 				}
 				else
 					// no refineries reachable?
-					return null;
+					return this;
 			}
 			else if( unit.Facing != 64 )
 				return new Turn( 64 ) { NextActivity = this };
@@ -69,7 +69,7 @@ namespace OpenRa.Game.Traits.Activities
 				renderUnit.PlayCustomAnimation( self, "empty",
 					() => isDone = true );
 
-			return null;
+			return this;
 		}
 
 		public void Cancel(Actor self)
diff --git a/OpenRa.Game/Traits/Activities/DeployMcv.cs b/OpenRa.Game/Traits/Activities/DeployMcv.cs
index d452b9a0c9..74354ee6af 100755
--- a/OpenRa.Game/Traits/Activities/DeployMcv.cs
+++ b/OpenRa.Game/Traits/Activities/DeployMcv.cs
@@ -15,7 +15,7 @@ namespace OpenRa.Game.Traits.Activities
 				Sound.Play("build5.aud");
 				Game.world.Add( new Actor( Rules.UnitInfo["fact"], self.Location - new int2( 1, 1 ), self.Owner ) );
 			} );
-			return null;
+			return this;
 		}
 
 		public void Cancel( Actor self )
diff --git a/OpenRa.Game/Traits/Activities/Fly.cs b/OpenRa.Game/Traits/Activities/Fly.cs
index b2718a88c5..ca4a631ad6 100644
--- a/OpenRa.Game/Traits/Activities/Fly.cs
+++ b/OpenRa.Game/Traits/Activities/Fly.cs
@@ -38,7 +38,7 @@ namespace OpenRa.Game.Traits.Activities
 			self.CenterLocation += speed * -float2.FromAngle((float)angle);
 			self.Location = ((1 / 24f) * self.CenterLocation).ToInt2();
 
-			return null;
+			return this;
 		}
 
 		public void Cancel(Actor self) { isCanceled = true; NextActivity = null; }
@@ -74,7 +74,7 @@ namespace OpenRa.Game.Traits.Activities
 			self.CenterLocation += speed * -float2.FromAngle((float)angle);
 			self.Location = ((1 / 24f) * self.CenterLocation).ToInt2();
 
-			return null;
+			return this;
 		}
 
 		public void Cancel(Actor self) { isCanceled = true; NextActivity = null; }
@@ -103,7 +103,7 @@ namespace OpenRa.Game.Traits.Activities
 			self.Location = ((1 / 24f) * self.CenterLocation).ToInt2();
 
 			unit.Altitude += Math.Sign(targetAltitude - unit.Altitude);
-			return null;
+			return this;
 		}
 
 		public void Cancel(Actor self) { remainingTicks = 0; NextActivity = null; }
diff --git a/OpenRa.Game/Traits/Activities/Follow.cs b/OpenRa.Game/Traits/Activities/Follow.cs
index fc4877ba8c..e53ec7e22d 100644
--- a/OpenRa.Game/Traits/Activities/Follow.cs
+++ b/OpenRa.Game/Traits/Activities/Follow.cs
@@ -24,7 +24,7 @@ namespace OpenRa.Game.Traits.Activities
 			if( !inRange )
 				return new Move( Target, Range ) { NextActivity = this };
 
-			return null;
+			return this;
 		}
 
 		public void Cancel(Actor self)
diff --git a/OpenRa.Game/Traits/Activities/Harvest.cs b/OpenRa.Game/Traits/Activities/Harvest.cs
index dfbc2023ff..677c34e953 100644
--- a/OpenRa.Game/Traits/Activities/Harvest.cs
+++ b/OpenRa.Game/Traits/Activities/Harvest.cs
@@ -12,49 +12,62 @@ namespace OpenRa.Game.Traits.Activities
 			var unit = self.traits.Get<Unit>();
 			var mobile = self.traits.Get<Mobile>();
 
-			if( isHarvesting ) return null;
-
-			if( NextActivity != null )
-				return NextActivity;
+			if( isHarvesting ) return this;
+			if( NextActivity != null ) return NextActivity;
 
 			var harv = self.traits.Get<Harvester>();
 
 			if( harv.IsFull )
 				return new DeliverOre { NextActivity = NextActivity };
 
-			var isGem = false;
-			if( Rules.Map.ContainsResource( self.Location ) &&
-				Rules.Map.Harvest( self.Location, out isGem ) )
-			{
-				var harvestAnim = "harvest" + Util.QuantizeFacing( unit.Facing, 8 );
-				var renderUnit = self.traits.WithInterface<RenderUnit>().First();	/* better have one of these! */
-				if( harvestAnim != renderUnit.anim.CurrentSequence.Name )
-				{
-					isHarvesting = true;
-					renderUnit.PlayCustomAnimation( self, harvestAnim, () => isHarvesting = false );
-				}
-				harv.AcceptResource( isGem );
-				return null;
-			}
+			if (HarvestThisTile(self))
+				return this;
 			else
 			{
-				self.QueueActivity( new Move(
-					() =>
-					{
-						var search = new PathSearch
-						{
-							heuristic = loc => ( Rules.Map.ContainsResource( loc ) ? 0 : 1 ),
-							umt = UnitMovementType.Wheel,
-							checkForBlocked = true
-						};
-						search.AddInitialCell( self.Location );
-						return Game.PathFinder.FindPath( search );
-					} ) );
-				self.QueueActivity( new Harvest() );
+				FindMoreOre(self);
 				return NextActivity;
 			}
 		}
 
+		bool HarvestThisTile(Actor self)
+		{
+			var unit = self.traits.Get<Unit>();
+			var harv = self.traits.Get<Harvester>();
+			var renderUnit = self.traits.WithInterface<RenderUnit>().First();	/* better have one of these! */
+
+			var isGem = false;
+			if (!Rules.Map.ContainsResource(self.Location) ||
+				!Rules.Map.Harvest(self.Location, out isGem))
+				return false;
+
+			var harvestAnim = "harvest" + Util.QuantizeFacing(unit.Facing, 8);
+
+			if (harvestAnim != renderUnit.anim.CurrentSequence.Name)
+			{
+				isHarvesting = true;
+				renderUnit.PlayCustomAnimation(self, harvestAnim, () => isHarvesting = false);
+			}
+			harv.AcceptResource(isGem);
+			return true;
+		}
+
+		void FindMoreOre(Actor self)
+		{
+			self.QueueActivity(new Move(
+				() =>
+				{
+					var search = new PathSearch
+					{
+						heuristic = loc => (Rules.Map.ContainsResource(loc) ? 0 : 1),
+						umt = UnitMovementType.Wheel,
+						checkForBlocked = true
+					};
+					search.AddInitialCell(self.Location);
+					return Game.PathFinder.FindPath(search);
+				}));
+			self.QueueActivity(new Harvest());
+		}
+
 		public void Cancel(Actor self) { }
 	}
 }
diff --git a/OpenRa.Game/Traits/Activities/Idle.cs b/OpenRa.Game/Traits/Activities/Idle.cs
new file mode 100644
index 0000000000..129bd7f6ec
--- /dev/null
+++ b/OpenRa.Game/Traits/Activities/Idle.cs
@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace OpenRa.Game.Traits.Activities
+{
+	class Idle : IActivity
+	{
+		public IActivity NextActivity { get; set; }
+
+		public IActivity Tick(Actor self) { return NextActivity; }
+		public void Cancel(Actor self) {}
+	}
+}
diff --git a/OpenRa.Game/Traits/Activities/Move.cs b/OpenRa.Game/Traits/Activities/Move.cs
index 9a655495a1..a1cfb0655c 100755
--- a/OpenRa.Game/Traits/Activities/Move.cs
+++ b/OpenRa.Game/Traits/Activities/Move.cs
@@ -59,7 +59,7 @@ namespace OpenRa.Game.Traits.Activities
 			if( move != null )
 			{
 				move.TickMove( self, mobile, this );
-				return null;
+				return this;
 			}
 
 			if( destination == self.Location )
@@ -74,14 +74,14 @@ namespace OpenRa.Game.Traits.Activities
 			if( path.Count == 0 )
 			{
 				destination = mobile.toCell;
-				return null;
+				return this;
 			}
 
 			destination = path[ 0 ];
 
 			var nextCell = PopPath( self, mobile );
 			if( nextCell == null )
-				return null;
+				return NextActivity;
 
 			int2 dir = nextCell.Value - mobile.fromCell;
 			var firstFacing = Util.GetFacing( dir, unit.Facing );
@@ -103,7 +103,7 @@ namespace OpenRa.Game.Traits.Activities
 
 				move.TickMove( self, mobile, this );
 
-				return null;
+				return this;
 			}
 		}
 
diff --git a/OpenRa.Game/Traits/Activities/Turn.cs b/OpenRa.Game/Traits/Activities/Turn.cs
index 53d22b822b..f07033d93e 100755
--- a/OpenRa.Game/Traits/Activities/Turn.cs
+++ b/OpenRa.Game/Traits/Activities/Turn.cs
@@ -20,7 +20,7 @@ namespace OpenRa.Game.Traits.Activities
 				return NextActivity;
 
 			Util.TickFacing( ref unit.Facing, desiredFacing, self.Info.ROT );
-			return null;
+			return this;
 		}
 
 		public void Cancel( Actor self )