diff --git a/OpenRA.Mods.Common/Traits/BotModules/UnitBuilderBotModule.cs b/OpenRA.Mods.Common/Traits/BotModules/UnitBuilderBotModule.cs
index 386448939b..7417e72c01 100644
--- a/OpenRA.Mods.Common/Traits/BotModules/UnitBuilderBotModule.cs
+++ b/OpenRA.Mods.Common/Traits/BotModules/UnitBuilderBotModule.cs
@@ -49,8 +49,7 @@ namespace OpenRA.Mods.Common.Traits
 		readonly List<string> queuedBuildRequests = new List<string>();
 
 		IBotRequestPauseUnitProduction[] requestPause;
-
-		List<Actor> idleUnits = new List<Actor>();
+		int idleUnitCount;
 
 		int ticks;
 
@@ -68,7 +67,7 @@ namespace OpenRA.Mods.Common.Traits
 
 		void IBotNotifyIdleBaseUnits.UpdatedIdleBaseUnits(List<Actor> idleUnits)
 		{
-			this.idleUnits = idleUnits;
+			idleUnitCount = idleUnits.Count;
 		}
 
 		void IBotTick.BotTick(IBot bot)
@@ -88,7 +87,7 @@ namespace OpenRA.Mods.Common.Traits
 				}
 
 				foreach (var q in Info.UnitQueues)
-					BuildUnit(bot, q, idleUnits.Count < Info.IdleBaseUnitsMaximum);
+					BuildUnit(bot, q, idleUnitCount < Info.IdleBaseUnitsMaximum);
 			}
 		}
 
@@ -218,7 +217,7 @@ namespace OpenRA.Mods.Common.Traits
 			return new List<MiniYamlNode>()
 			{
 				new MiniYamlNode("QueuedBuildRequests", FieldSaver.FormatValue(queuedBuildRequests.ToArray())),
-				new MiniYamlNode("IdleUnits", FieldSaver.FormatValue(idleUnits.Select(a => a.ActorID).ToArray()))
+				new MiniYamlNode("IdleUnitCount", FieldSaver.FormatValue(idleUnitCount))
 			};
 		}
 
@@ -234,13 +233,9 @@ namespace OpenRA.Mods.Common.Traits
 				queuedBuildRequests.AddRange(FieldLoader.GetValue<string[]>("QueuedBuildRequests", queuedBuildRequestsNode.Value.Value));
 			}
 
-			var idleUnitsNode = data.FirstOrDefault(n => n.Key == "IdleUnits");
-			if (idleUnitsNode != null)
-			{
-				idleUnits.Clear();
-				idleUnits.AddRange(FieldLoader.GetValue<uint[]>("IdleUnits", idleUnitsNode.Value.Value)
-					.Select(a => world.GetActorById(a)));
-			}
+			var idleUnitCountNode = data.FirstOrDefault(n => n.Key == "IdleUnitCount");
+			if (idleUnitCountNode != null)
+				idleUnitCount = FieldLoader.GetValue<int>("IdleUnitCount", idleUnitCountNode.Value.Value);
 		}
 	}
 }