diff --git a/OpenRA.Mods.Common/EditorBrushes/EditorDefaultBrush.cs b/OpenRA.Mods.Common/EditorBrushes/EditorDefaultBrush.cs
index 5a8178192a..d8cf8430ba 100644
--- a/OpenRA.Mods.Common/EditorBrushes/EditorDefaultBrush.cs
+++ b/OpenRA.Mods.Common/EditorBrushes/EditorDefaultBrush.cs
@@ -65,10 +65,11 @@ namespace OpenRA.Mods.Common.Widgets
 			var underCursor = editorLayer.PreviewsAt(worldRenderer.Viewport.ViewToWorldPx(mi.Location))
 				.FirstOrDefault();
 
+			var mapResources = world.Map.MapResources.Value;
 			ResourceType type;
 			if (underCursor != null)
 				editorWidget.SetTooltip(underCursor.Tooltip);
-			else if (world.Map.Contains(cell) && resources.TryGetValue(world.Map.MapResources.Value[cell].Type, out type))
+			else if (mapResources.Contains(cell) && resources.TryGetValue(mapResources[cell].Type, out type))
 				editorWidget.SetTooltip(type.Info.Name);
 			else
 				editorWidget.SetTooltip(null);
@@ -84,8 +85,8 @@ namespace OpenRA.Mods.Common.Widgets
 				if (underCursor != null)
 					editorLayer.Remove(underCursor);
 
-				if (world.Map.MapResources.Value[cell].Type != 0)
-					world.Map.MapResources.Value[cell] = new ResourceTile();
+				if (mapResources.Contains(cell) && mapResources[cell].Type != 0)
+					mapResources[cell] = new ResourceTile();
 			}
 			else if (mi.Button == MouseButton.Left && mi.Event == MouseInputEvent.Down)
 			{
diff --git a/OpenRA.Mods.Common/EditorBrushes/EditorResourceBrush.cs b/OpenRA.Mods.Common/EditorBrushes/EditorResourceBrush.cs
index bfd73fe4f6..965d4285c6 100644
--- a/OpenRA.Mods.Common/EditorBrushes/EditorResourceBrush.cs
+++ b/OpenRA.Mods.Common/EditorBrushes/EditorResourceBrush.cs
@@ -82,7 +82,8 @@ namespace OpenRA.Mods.Common.Widgets
 
 		public bool AllowResourceAt(CPos cell)
 		{
-			if (!world.Map.Contains(cell))
+			var mapResources = world.Map.MapResources.Value;
+			if (!mapResources.Contains(cell))
 				return false;
 
 			var tile = world.Map.MapTiles.Value[cell];
@@ -92,7 +93,7 @@ namespace OpenRA.Mods.Common.Widgets
 
 			var terrainType = world.TileSet.TerrainInfo[tileInfo.TerrainType];
 
-			if (world.Map.MapResources.Value[cell].Type == ResourceType.ResourceType)
+			if (mapResources[cell].Type == ResourceType.ResourceType)
 				return false;
 
 			if (!ResourceType.AllowedTerrainTypes.Contains(terrainType.Type))
diff --git a/OpenRA.Mods.Common/Traits/World/EditorResourceLayer.cs b/OpenRA.Mods.Common/Traits/World/EditorResourceLayer.cs
index 1c6ba077f5..f90f549466 100644
--- a/OpenRA.Mods.Common/Traits/World/EditorResourceLayer.cs
+++ b/OpenRA.Mods.Common/Traits/World/EditorResourceLayer.cs
@@ -86,11 +86,7 @@ namespace OpenRA.Mods.Common.Traits
 			// so we must also touch all the neighbouring tiles
 			Dirty.Add(cell);
 			foreach (var d in CVec.Directions)
-			{
-				var c = cell + d;
-				if (Map.Contains(c))
-					Dirty.Add(c);
-			}
+				Dirty.Add(cell + d);
 		}
 
 		protected virtual string ChooseRandomVariant(ResourceType t)
@@ -103,10 +99,16 @@ namespace OpenRA.Mods.Common.Traits
 			// Set density based on the number of neighboring resources
 			var adjacent = 0;
 			var type = Tiles[c].Type;
+			var resources = Map.MapResources.Value;
 			for (var u = -1; u < 2; u++)
+			{
 				for (var v = -1; v < 2; v++)
-					if (Map.MapResources.Value[c + new CVec(u, v)].Type == type.Info.ResourceType)
+				{
+					var cell = c + new CVec(u, v);
+					if (resources.Contains(cell) && resources[cell].Type == type.Info.ResourceType)
 						adjacent++;
+				}
+			}
 
 			return Math.Max(int2.Lerp(0, type.Info.MaxDensity, adjacent, 9), 1);
 		}
@@ -139,7 +141,8 @@ namespace OpenRA.Mods.Common.Traits
 				return;
 
 			foreach (var c in Dirty)
-				Tiles[c] = UpdateDirtyTile(c);
+				if (Tiles.Contains(c))
+					Tiles[c] = UpdateDirtyTile(c);
 
 			Dirty.Clear();
 
diff --git a/OpenRA.Mods.Common/Traits/World/ResourceLayer.cs b/OpenRA.Mods.Common/Traits/World/ResourceLayer.cs
index 84a16d522a..5827cdb329 100644
--- a/OpenRA.Mods.Common/Traits/World/ResourceLayer.cs
+++ b/OpenRA.Mods.Common/Traits/World/ResourceLayer.cs
@@ -50,9 +50,14 @@ namespace OpenRA.Mods.Common.Traits
 		{
 			var sum = 0;
 			for (var u = -1; u < 2; u++)
+			{
 				for (var v = -1; v < 2; v++)
-					if (content[cell + new CVec(u, v)].Type == t)
+				{
+					var c = cell + new CVec(u, v);
+					if (content.Contains(c) && content[c].Type == t)
 						++sum;
+				}
+			}
 
 			return sum;
 		}
diff --git a/OpenRA.Mods.D2k/Traits/World/D2kEditorResourceLayer.cs b/OpenRA.Mods.D2k/Traits/World/D2kEditorResourceLayer.cs
index f46354380f..0c2f3cb538 100644
--- a/OpenRA.Mods.D2k/Traits/World/D2kEditorResourceLayer.cs
+++ b/OpenRA.Mods.D2k/Traits/World/D2kEditorResourceLayer.cs
@@ -65,31 +65,36 @@ namespace OpenRA.Mods.D2k.Traits
 			return D2kResourceLayer.Variants.Keys.Random(Game.CosmeticRandom);
 		}
 
+		bool CellContains(CPos c, ResourceType t)
+		{
+			return Tiles.Contains(c) && Tiles[c].Type == t;
+		}
+
 		ClearSides FindClearSides(ResourceType t, CPos p)
 		{
 			var ret = ClearSides.None;
-			if (Tiles[p + new CVec(0, -1)].Type != t)
+			if (!CellContains(p + new CVec(0, -1), t))
 				ret |= ClearSides.Top | ClearSides.TopLeft | ClearSides.TopRight;
 
-			if (Tiles[p + new CVec(-1, 0)].Type != t)
+			if (!CellContains(p + new CVec(-1, 0), t))
 				ret |= ClearSides.Left | ClearSides.TopLeft | ClearSides.BottomLeft;
 
-			if (Tiles[p + new CVec(1, 0)].Type != t)
+			if (!CellContains(p + new CVec(1, 0), t))
 				ret |= ClearSides.Right | ClearSides.TopRight | ClearSides.BottomRight;
 
-			if (Tiles[p + new CVec(0, 1)].Type != t)
+			if (!CellContains(p + new CVec(0, 1), t))
 				ret |= ClearSides.Bottom | ClearSides.BottomLeft | ClearSides.BottomRight;
 
-			if (Tiles[p + new CVec(-1, -1)].Type != t)
+			if (!CellContains(p + new CVec(-1, -1), t))
 				ret |= ClearSides.TopLeft;
 
-			if (Tiles[p + new CVec(1, -1)].Type != t)
+			if (!CellContains(p + new CVec(1, -1), t))
 				ret |= ClearSides.TopRight;
 
-			if (Tiles[p + new CVec(-1, 1)].Type != t)
+			if (!CellContains(p + new CVec(-1, 1), t))
 				ret |= ClearSides.BottomLeft;
 
-			if (Tiles[p + new CVec(1, 1)].Type != t)
+			if (!CellContains(p + new CVec(1, 1), t))
 				ret |= ClearSides.BottomRight;
 
 			return ret;
diff --git a/OpenRA.Mods.D2k/Traits/World/D2kResourceLayer.cs b/OpenRA.Mods.D2k/Traits/World/D2kResourceLayer.cs
index dce447e39c..e63d1ea45d 100644
--- a/OpenRA.Mods.D2k/Traits/World/D2kResourceLayer.cs
+++ b/OpenRA.Mods.D2k/Traits/World/D2kResourceLayer.cs
@@ -96,31 +96,36 @@ namespace OpenRA.Mods.D2k.Traits
 			{ ClearSides.Bottom | ClearSides.TopLeft | ClearSides.BottomLeft | ClearSides.BottomRight, 49 },
 		};
 
+		bool CellContains(CPos c, ResourceType t)
+		{
+			return render.Contains(c) && render[c].Type == t;
+		}
+
 		ClearSides FindClearSides(ResourceType t, CPos p)
 		{
 			var ret = ClearSides.None;
-			if (render[p + new CVec(0, -1)].Type != t)
+			if (!CellContains(p + new CVec(0, -1), t))
 				ret |= ClearSides.Top | ClearSides.TopLeft | ClearSides.TopRight;
 
-			if (render[p + new CVec(-1, 0)].Type != t)
+			if (!CellContains(p + new CVec(-1, 0), t))
 				ret |= ClearSides.Left | ClearSides.TopLeft | ClearSides.BottomLeft;
 
-			if (render[p + new CVec(1, 0)].Type != t)
+			if (!CellContains(p + new CVec(1, 0), t))
 				ret |= ClearSides.Right | ClearSides.TopRight | ClearSides.BottomRight;
 
-			if (render[p + new CVec(0, 1)].Type != t)
+			if (!CellContains(p + new CVec(0, 1), t))
 				ret |= ClearSides.Bottom | ClearSides.BottomLeft | ClearSides.BottomRight;
 
-			if (render[p + new CVec(-1, -1)].Type != t)
+			if (!CellContains(p + new CVec(-1, -1), t))
 				ret |= ClearSides.TopLeft;
 
-			if (render[p + new CVec(1, -1)].Type != t)
+			if (!CellContains(p + new CVec(1, -1), t))
 				ret |= ClearSides.TopRight;
 
-			if (render[p + new CVec(-1, 1)].Type != t)
+			if (!CellContains(p + new CVec(-1, 1), t))
 				ret |= ClearSides.BottomLeft;
 
-			if (render[p + new CVec(1, 1)].Type != t)
+			if (!CellContains(p + new CVec(1, 1), t))
 				ret |= ClearSides.BottomRight;
 
 			return ret;
@@ -128,6 +133,9 @@ namespace OpenRA.Mods.D2k.Traits
 
 		void UpdateRenderedTileInner(CPos p)
 		{
+			if (!render.Contains(p))
+				return;
+
 			var t = render[p];
 			if (t.Density > 0)
 			{